ci: Add GitHub Actions

[crawfxrd]

TODO:

• Determine cache storage usage/cost
  • coreboot toolchain is using 160 MB, and apparently there's 10 GB available
• Determine artifact storage usage/cost
  • disabled upload-artifact for now because it's going to be a lot of space
• Replace script in private firmware repo for downloading build artifacts when staging updates
  • depends on uploading artifacts
• Investigate reducing CI time, esp. on cloning all the submodules and installing build dependencies
• Limit the models built by default to not hit concurrency limit?
  • But would want a way to manually trigger a build by specifying models like Jenkins currently does

[crawfxrd]

Received non-retryable error: Failed request: (409) Conflict: cache entry with the same key, version, and scope already exists

The fuck you mean?

[crawfxrd]

usb.img is removed from the build artifacts as it takes up a lot of space (at least doubling the size of the output) but is not normally used. It would still be >300 MiB for the artifacts per commit, so I've disabled use of actions/upload-artifact.

UEFIPAYLOAD.fd could be removed to reduce artifacts by another 8 MiB each, as this is already included in firmware.rom.