Add Arch Linux to the CI test matrix

[twangboy]

Summary

Arch Linux has had full install support in bootstrap-salt.sh for a while (f706f4d recently ported ALT Linux's onedir fixes over to it), but it has never had any CI coverage — it doesn't appear anywhere in .github/workflows/templates/generate.py, so no archlinux job exists in the matrix. f706f4d's own commit message notes it "couldn't be verified end-to-end via CI".

This PR:

1. Fixes a real bug in install_arch_linux_onedir_post(): it only ever created/enabled a hardcoded salt-minion.service unit. install_arch_linux_restart_daemons()/install_arch_check_services() are generic and unconditionally loop over api/master/minion/syndic per the _INSTALL_* flags, so a -M (master) or -S (syndic) onedir install on Arch would fail trying to restart/check a unit that was never created. Ported ALT Linux's install_alt_linux_onedir_post() loop, which creates a unit per requested role. Default minion-only behavior is unchanged.
2. Wires Arch Linux into the CI matrix, following the same pattern used to add ALT Linux: archlinux added to generate.py's LINUX_DISTROS/STABLE_DISTROS/ONEDIR_DISTROS, DISTRO_DISPLAY_NAMES, and CONTAINER_SLUG_NAMES (rolling-release, so distro-slug == container-slug == archlinux), ci.yml regenerated via pre-commit run -av generate-actions-workflow, and archlinux added to test-linux.yml's --break-system-packages conditionals (Arch enforces the same PEP 668 restriction as Debian≥12/Ubuntu≥24).

Scope notes / follow-ups expected

• No version blacklist entries added yet — full stable+git+onedir matrix, matching ALT Linux's own initial rollout. Two likely candidates to watch on the first real run and blacklist/fix as follow-ups if they fail:
  • install_arch_linux_stable() builds from the current AUR salt PKGBUILD with no version pinning, so stable-* instances may not resolve to the requested Salt version.
  • git-* instances may hit Arch's PEP 668 restriction inside bootstrap-salt.sh itself, since _USE_BREAK_SYSTEM_PACKAGES only covers ubuntu>=24/debian>=12, not Arch.
• test-linux.yml invokes bootstrap-salt.sh <type> <version> with no -M/-S flags for any distro, so the onedir_post fix (item 1) won't itself be exercised by this new CI matrix — it's still correct/necessary for real-world -M/-S usage, matching ALT Linux's own current CI limitation.

Testing

• bash -n bootstrap-salt.sh — clean.
• pre-commit run -av generate-actions-workflow — idempotent, diff matches expectations (new archlinux job inserted alphabetically, instance list identical to ubuntu-2604's unblacklisted reference, added to set-pipeline-exit-status needs).
• YAML-validated the regenerated ci.yml and edited test-linux.yml.
• Manually traced install_arch_linux_onedir_post()'s new per-role loop against _INSTALL_MASTER/_INSTALL_MINION/_INSTALL_SYNDIC/_START_DAEMONS defaults and flag semantics (confirmed in the arg-parsing section) — default (minion-only) behavior unchanged, -M/-S now produce their own units.
• No local docker/shellcheck available to run the repo's shellcheck hook; the rewritten function is a structural copy of install_alt_linux_onedir_post(), which already passes CI's shellcheck.

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