Release Automation

[StarWitch]

A lot of trial-and-error went into this, but here it is.

General steps in this:

1. Create a v*.*.*[-alpha/-rc/-etc] branch, wait for the Release Builds job to finish, download the artifacts on the job page in Actions (near the bottom of the page) (or use the API), as well as the build logs, verify them.
2. If everything looks good, push to the tag (i.e refs/tags/v*.*.*[-alpha/-rc/-etc]).
3. It will build the files AGAIN, and you can double-check them there too.
4. Upload them as usual to the official tag release page. (At least until the Publishing pipeline is also reviewed/merged, split off from this one.)

Most of what's present is already done in the CI pipeline, but using different machine files to release a more optimized build, as well as extra steps for packaging. There's a little bit of duplication between CI and Release, but it's easier to read and maintain than trying to write a bunch of conditionals inside a single .yaml file to do both CI and release builds.

I've also made it so it produces .sig and .sha256 files for each one of the releases, and added some Python scripting to help that along.

Here's what it looks like in action: https://github.com/StarWitch/taisei/releases/tag/v1.3.50 (ignore the v1.3.50, it's just for testing)

Post-Merge Steps

1. Adding a GPG_PRIVATE_KEY and GPG_PASSPHRASE to Settings -> Secrets inside the main Taisei project so the pipeline can generate .sig files. These files are hidden in normal pipeline use and I've made sure to fork our version of ghaction-import-gpg at taisei-project/ghaction-import-gpg.

[Akaricchi]

Not a thorough review, just addressing what sticks out the most for now.

[StarWitch]

Spent the afternoon refactoring this. sha256sum and sig files are now generated by the new releasefiles.py script which meson now calls directly with release_files provided as an argument. I also found a better releasing script (the one I linked before) which allows wildcards, which is much simpler to use. I also forked the GPG action like you suggested, since that's safer/easier. Let me know if this is in the right direction.

[StarWitch]

A bunch of changes since I last updated this PR.

1. Updated all of the action versions to get rid of as many deprecation warnings as possible.
2. Updated to ubuntu-20.04 since ubuntu-18.04 is being removed from GitHub Actions in six months. We could roll our own but it was deemed not worth it.
3. Updated macos-11 to macos-12.
4. Allow specifying of an arbitrary commit through workflow_dispatch.
5. Added the Linux library linker checking script to the Linux stage.
6. Some minor optimizations.

[Akaricchi]

Everything looks good to land modulo comments. I hope this is my last review here. We can keep improving it for a long time, but this PR has been in the oven for way too long already. Great work!

[StarWitch]

Remaining comments resolved.

There's currently an issue with macOS builds due to this issue, but the fix for that was merged and it should be deployed to the runner pool soon, which shouldn't require editing the workflow at all.