Migrate from Buildkite to GitHub actions

[viktor-ferenczi]

• Introduced GitHub workflow
• Removed Buildkite script
• ️Replaced the Buildkite specific logos with emojis

[viktor-ferenczi]

The following build targets may fail with a misleading error 2 (file not found) error:

• Musl (dynamic) build images
• Musl (static) build images
• Cross compilation images

While the issue appears to be on an rm -rf command the actual problem is not there. The issue is caused by this download timing out:

curl -C - -L -o config.sub "http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.sub;hb=3d5db9ebe860"

This happens when git.savannah.gnu.org is down, which you can check on https://status.doomemacs.org/