fix: make AUR packaging x86_64-only and stabilize local validation

Turns out Arch Linux doesn't officially support aarch64 architecture, so we will drop if from this AUR build process.

Changes:

- drop aarch64 from PKGBUILD, .SRCINFO, and AUR validation workflow
- keep AUR process aligned with official Arch Linux x86_64 support
- install rust directly in CI to avoid Arch cargo provider prompts
- fetch sources before running cargo audit and audit inside the
fetched repo
- disable makepkg LTO for this package to avoid Arch packaging link
failures
- mark /etc/numa.toml as a backup file
- Add local AUR build scratch directory exclusion to .gitignore

.SRCINFO

@@ -4,14 +4,15 @@ pkgbase = numa-git
 	pkgrel = 1
 	url = https://github.com/razvandimescu/numa
 	arch = x86_64
-	arch = aarch64
 	license = MIT
+	options = !lto
 	makedepends = cargo
 	makedepends = git
 	depends = gcc-libs
 	depends = glibc
 	provides = numa
 	conflicts = numa
+	backup = etc/numa.toml
 	source = numa::git+https://github.com/razvandimescu/numa.git
 	sha256sums = SKIP