Adjust GitHub Actions to "git fetch" the current repository's history…

[tianon]

… into the Bashbrew cache when the commits appear to be unreachable (such as on a PR branch, for example)

before:

failed fetching/scraping FROM for "busybox" (tags "glibc", arch "amd64")
failed fetching Git repo for arch "amd64" from entry "Maintainers: foo (@bar)\nTags: glibc\nArchitectures: amd64\nGitRepo: https://github.com/docker-library/busybox.git\nGitFetch: refs/heads/master\nGitCommit: eb35037646377a3a5715bbc1a39fc49cae222a04\nDirectory: glibc\nFile: Dockerfile"
object not found

after:

Processing busybox:glibc ...
Processing busybox:musl ...
Processing busybox:uclibc ...

Also, this includes a minor adjustment to the handling of multi-architecture such that we prefer either the current architecture (typically amd64), or the first supported architecture for the purposes of determining the appropriate build context.