Guard three x86-only assumptions so Omarchy can install on aarch64 - #8039

Three unconditional assumptions in the install path only hold on x86_64. None is
x86-specific in spirit, and each one blocks or breaks an ARM64 install. All three were
found by actually installing Omarchy on aarch64 (Arch Linux ARM) and are fixed the same
way: derive from uname -m instead of assuming.

1. etc/mkinitcpio.conf.d/thunderbolt_module.conf

MODULES+=(thunderbolt) is unconditional, but Thunderbolt is x86-oriented hardware and the
module isn't built for every architecture. ALARM's aarch64 kernel has no thunderbolt, and
mkinitcpio treats an unresolvable MODULES entry as a hard error, so every initramfs
build fails:

==> ERROR: module not found: `thunderbolt'

No initramfs means no UKI, no boot entry, and an install that completes and then can't boot.
Because bootloader setup is the last phase, the failure surfaces as "no operating system
installed" at the firmware, which points at the wrong end of the process entirely.

2. install/post-install/pacman.sh

This overwrites /etc/pacman.conf and the mirrorlist unconditionally with files that
describe an x86_64 Arch system: [multilib] is 32-bit x86 libraries and exists on no ARM
mirror, and the mirrorlist points at Omarchy's mirror of Arch, which is x86_64 only. Arch
Linux ARM also lays its tree out as $arch/$repo rather than Arch's $repo/os/$arch, so
the mirrorlist can't simply be pointed at a different host.

The first revision of this PR skipped the restore on aarch64. That was wrong: at this point
the target still carries the live ISO's pacman.conf, which knows only the offline mirror,
and that directory doesn't exist on the installed system. Skipping left every aarch64
install unable to run pacman at all.

This revision derives the config from Omarchy's template instead: drop [multilib], keep
[omarchy] (its $arch placeholder resolves correctly), add ALARM's [alarm] and [aur]
repositories, and leave the mirrorlist the distribution installed. The [options] block
and everything else in the template come through unchanged, so OMARCHY_MIRROR still
selects stable/edge/rc as it does on x86.

Known gap, not addressed here: pkgs.omarchy.org has no aarch64 tree yet
(omacom/omarchy-pkgs#199), so [omarchy] 404s on ARM. pacman refuses all sync
transactions while a configured repository's database is missing, so until that tree is
published an aarch64 install can't pacman -S anything. The config written here is the
one that starts working the moment the tree exists; omitting [omarchy] would work today
and then never update Omarchy.

3. install/user/mise-work.sh

Node publishes its builds as linux-x64 / linux-arm64, which does not match
uname -m. The bundled-tarball lookup hardcodes linux-x64, so on ARM it finds nothing and
the install aborts:

failed phase: Staging provisioning: no bundled Node tarball in /opt/packages

The sed that parses the version back out of the filename needs the same treatment,
otherwise it silently yields the whole filename instead of the version.

Testing

On Arch Linux ARM aarch64, natively rather than under emulation:

• Each guard, both branches. thunderbolt_module.conf sourced with uname -m faked to
  each architecture yields MODULES=(thunderbolt) on x86_64 and empty on aarch64, and a
  real mkinitcpio run with the guarded file succeeds on a kernel without the module.
  mise-work.sh's lookup and sed were run against node-v24.7.0-linux-{x64,arm64}.tar.gz
  under both architectures; each finds its own tarball and extracts 24.7.0, and the
  upstream x86 sed gives the same answer on the x64 file.

• pacman.sh, sandboxed, all three mirrors. With /etc redirected, the script was run
  for stable, edge and rc on aarch64: [multilib] gone, [options] byte-identical to
  the template, the mirrorlist untouched, and pacman-conf --repo-list parses the result as
  core extra omarchy alarm aur. With uname -m faked to x86_64 the output is
  byte-identical to the plain cp of both files. A live pacman -Sy against the stable
  output synced core, extra, alarm and aur from ALARM's mirror; only omarchy
  failed, with the 404 described above.

• Full install. omarchy and omarchy-settings packages built from this branch went
  into an ISO (Build the ISO for the host architecture, adding aarch64 support omarchy-iso#121), which installed to an encrypted target and
  booted unattended into the desktop. On the installed system /etc/pacman.conf is the
  derived one, the mirrorlist is the one ALARM's pacman-mirrorlist installed, and
  pacman -Sy syncs core, extra, alarm and aur with no errors; the only failure is
  omarchy.db, the 404 described above.

  The first run of this test also showed the installer prepending three x86 Arch mirrors to
  the target's mirrorlist, so every sync walked through a dozen 404s before reaching ALARM's.
  That is the ISO's configurator, not this repository, and is fixed in omarchy-iso#121.

All three files pass bash -n and ShellCheck. No behaviour change on x86_64: each guard
takes the existing path there, and pacman.sh's x86_64 branch is the unchanged two-line copy.