[WIP] Add GitHub Actions workflow to build releases

[pR0Ps]

Will automatically build and release versions when tags like 'x.x.x' are pushed. Can also be run manually to produce a build artifact.

Note that the build process is a bit hacky:

• It uses the Ubuntu image provided by GitHub, but adds the Debian bullseye repo in order to get more recent packages.
• It edits some of the files that cmake generates so that the host's qt system's moc and rcc build tools are used instead of the mingw-provided ones (which are win32 executables).
• It builds and installs pacman so it can use it to download and install the required mingw packages.

Links to results:

• Result of manually running the workflow (builds an artifact):
• Result of the workflow running when a tag is pushed (publishes a release)

[pR0Ps]

Currently there is an issue where invader-edit-qt.exe won't run due to a library error.

@SnowyMouse I'll leave it up to you how you want to proceed with this.

I've tried a bunch of things and couldn't figure out how to make it work. My hope is that this PR can serve as a starting point for someone more well-versed in building this project than me.

Some options:

1. Fix the build. It's entirely possible that I've done this entirely wrong and the solution is obvious to someone more knowledgeable than me.
2. Since this project builds easily and without error on Arch Linux you could run a self-hosted GitHub runner on Arch and have it do the build instead of using GitHub's Ubuntu image on their infrastructure.