ci: pin rust version

[adil192]

This PR uses a pinned version of Rust instead of just using the latest stable. This helps make builds more reproducible.

TODO:

• Patch dependencies to use specific Rust version like in F-Droid's build manifest
• Use GitHub Actions to check if Rust needs updating
• Validate RUST_VERSION in patches/patch_rust_versions.sh to prevent command injection
• Make sure this doesn't mess up the snap build (see snap: update to core24 and cleanup #1561)
  • ci: build snap without rust #1564