fix: preserve latest binary executable mode

[MukundaKatta]

Summary

• package the latest-commit Linux binary as a tar.zst archive instead of a raw zstd-compressed file
• preserve the executable permission bit when users extract the archive
• update the README wording to describe the latest-commit download as an archive

Fixes #277.

Tests

• local archive check: tar -C <dir> -cf - sed | zstd -19 -o sed-x86_64-unknown-linux-gnu.tar.zst, then zstd -dc ... | tar -xf - and test -x sed
• git diff --check
• cargo test test_version
• cargo test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.20%. Comparing base (f083fa0) to head (a7c98bd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #434   +/-   ##
=======================================
  Coverage   82.20%   82.20%           
=======================================
  Files          13       13           
  Lines        5542     5542           
  Branches      310      310           
=======================================
  Hits         4556     4556           
  Misses        983      983           
  Partials        3        3           

Flag	Coverage Δ
macos_latest	82.89% <ø> (ø)
ubuntu_latest	82.99% <ø> (ø)
windows_latest	0.00% <ø> (ø)

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[oech3]

There are 2 archives now. Needed to manually remove old one.