fix(bootstrap): port ALT Linux onedir fixes to Arch Linux

[twangboy]

What does this PR do?

install_arch_linux_onedir()/install_arch_linux_onedir_post() carry the same three bugs just fixed for ALT Linux (its code was evidently copied from Arch's originally):

1. Bare major version isn't resolved. ONEDIR_REV can legitimately be a bare 4-digit major version (e.g. "3006") - a documented, valid input per __validate_salt_version_arg. It passed through unresolved and produced a nonexistent GitHub release tag (v3006).
2. GitHub Releases doesn't carry an onedir tarball asset for every historical point release. E.g. v3007.1's release page exists but only has source tarballs/docs/epub, no onedir asset - only newer releases got onedir assets attached to GitHub Releases.
3. Wrong arm64 filename. arch="aarch64" was used, but onedir tarball filenames (on both GitHub and Broadcom's artifactory) use arm64, not aarch64.
4. install_arch_linux_onedir_post() only exports PATH via /etc/profile.d, which a non-login-shell invocation (e.g. docker exec <container> salt-call ..., exactly how this repo's own CI invokes tests) never sources.

Fix: reuse __get_packagesite_onedir_latest() (already used by the macOS/Windows/Photon onedir installers) to resolve latest/bare-major versions via the artifactory directory listing, fetch the tarball from the artifactory generic repo instead of GitHub Releases, fix the arm64 filename, and symlink onedir binaries into /usr/bin.

Verification

Arch Linux has no CI coverage in this repo - no arch/archlinux reference anywhere in .github/workflows/test-linux.yml or .github/workflows/templates/generate.py - which is exactly how these bugs went unnoticed while the identical ALT Linux code got caught immediately by its own CI matrix. Since this couldn't be verified end-to-end via CI, verified instead:

• bash -n bootstrap-salt.sh syntax check
• The version-resolution logic exercised in isolation (stubbed __get_packagesite_onedir_latest) against latest, bare-major (3006), full-minor (3007.13), and package-release-suffix (3008.1-1) inputs - all resolve and build URLs correctly
• The resulting artifactory URLs confirmed live (200) for a real version, both x86_64 and arm64

Given the lack of CI coverage, extra reviewer scrutiny (or a follow-up "add Arch Linux to the CI matrix" issue) is probably warranted here.

What issues does this PR fix or reference?

Same class of bug as salt-bootstrap#2125 (ALT Linux onedir fixes), applied to Arch Linux's pre-existing, structurally identical code.