Gummiboot: Switch to zip archive.

For some reason tarballs from cgit have a different hash each time you download.
Merge commit 'refs/top-bases/experimental/overlays' into experimental/overlays

# By Shea Levy
# Via Shea Levy
* commit 'refs/top-bases/experimental/overlays':
  Gummiboot: Switch to zip archive.
Merge branch 'experimental/overlays' into refs/top-bases/experimental/hack-nix

# By Shea Levy
# Via Marc Weber (1) and Shea Levy (1)
* experimental/overlays:
  Gummiboot: Switch to zip archive.
Merge commit 'refs/top-bases/experimental/hack-nix' into experimental/hack-nix

# By Shea Levy
# Via Marc Weber (2) and Shea Levy (1)
* commit 'refs/top-bases/experimental/hack-nix':
  Gummiboot: Switch to zip archive.