Investigate the use of git shallow clones (to build instead tarballs)

Our use case:

• We currently builds from tarballs
• We'd like to build from git for several different reasons
• Problem is that git repos can be much bigger than a simple tarball (for example, linux repo is ~2GB, 175MB with clone with --depth=1)

Downstream issue that depends on this: freedesktop-sdk/freedesktop-sdk#22