feat: syntax/style checks as well as tooling

Adds pre-commit hooks to check PKGBUILD syntax as well as style via shfmt. This should ensure basic integrity of the files we build.

Also adds deployment via GitLab (opt-in). Run it as follows:

• Add "[deploy all]" to the commit message to run a full garuda routine
• Add "[deploy pkgname]", eg. "[deploy garuda-bash-config]" to the commit message to build only one specific package

The builder has been prepared in infra-nix@120c1e13, using a very locked down ssh shell with only access to 2 actions.