pacman: add build scripts and Linux GitHub CI

[asiekierka]

Extracted from wonderful-packages and slightly tweaked to work outside of that build system. To be done on your end:

• Figure out a versioning scheme. If you're going to ingest Git builds into your own repository, you might want to either include the number of Git commits in the package's version number (so updates work), or even have separate Git and stable packages like I used to. I removed this from the PR for simplicity, but this is what you need to have multiple package names in a multi-package project, and this allows pulling in information from the Git repository.
• Windows builds. I'm not even trying to automate that, sorry.
• Configuring the on rules to match your project's needs.

[AntonioND]

I'm sure this will need some tweaks, but it's great to have this, thanks a lot!