build: standardize release asset naming

[conf]

A Pull Request should be associated with an Issue.

Related issue: #1728

Description

This PR reworks the release packaging so the published assets have stable,
predictable names that are easy to automate against (e.g. installing a fixed
version from a Dockerfile without first listing a release's assets), and
slims down the packaging tooling along the way.

Asset naming (build.sh)

• Drop the generated timestamp from tarball names — the biggest pain point,
  since it forced consumers to list a release's assets before they could pick a
  download.
• Drop the -binary infix from tarball names.
• Name macOS tarballs with darwin (matches GOOS) instead of osx.
• Drop the version from .rpm/.deb filenames — the release download URL
  already carries it (.../download/v1.1.10/gh-ost.x86_64.rpm). Package
  metadata still records the version and epoch.
• Build arm64 .rpm and .deb in addition to amd64, using each
  ecosystem's arch convention (x86_64/aarch64 for rpm, amd64/arm64
  for deb).
• Stop publishing the bare gh-ost binary — it was just the last build
  (darwin/arm64) left over in the build dir, and its name made the OS/arch
  non-obvious. Tarballs and packages already cover every OS/arch.
• Emit a SHA256SUMS manifest so a download can be verified with
  sha256sum -c SHA256SUMS (or --ignore-missing for a single file).

Resulting assets:

gh-ost-linux-amd64.tar.gz    gh-ost.x86_64.rpm    gh-ost.amd64.deb
gh-ost-linux-arm64.tar.gz    gh-ost.aarch64.rpm   gh-ost.arm64.deb
gh-ost-darwin-amd64.tar.gz
gh-ost-darwin-arm64.tar.gz   SHA256SUMS

Packaging tooling (Dockerfile.packaging, script/dock)

• Collapse the per-package RUN apt-get install layers into one, drop packages
  already provided by the golang:bookworm base (git, tar, curl, gcc, g++,
  bash) and the unused rsync, and drop the legacy GOPATH src/ layout (the
  build is module-mode). Only ruby/ruby-dev/build-essential (for fpm) and
  rpm (for rpmbuild) are installed now.
• Use a multi-stage build with a scratch artifacts stage so
  docker build --output type=local exports the assets straight to the host.
  This drops the intermediate container, bind mount, -it TTY and
  find | xargs cp step from script/dock pkg.
• Fix the stale/misleading usage text in script/dock.

Verification

Built all assets via script/dock pkg and verified: every tarball/package
contains a binary of the expected arch, the rpm/deb arch metadata is correct,
the deb version is 1:1.1.10 (epoch preserved), sha256sum -c SHA256SUMS
round-trips green, and the exported directory contains only the release assets.

⚠️ Note for the release-upload step: SHA256SUMS has no gh-ost
prefix, so it will not match a gh-ost* upload glob. The upload step needs to
include it explicitly (or upload the whole /tmp/gh-ost-release directory).

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[conf]

Thanks a lot! Any ETA for the release?