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/gimp-git' into experimental/gimp-git

# By Shea Levy
# Via Shea Levy
* commit 'refs/top-bases/experimental/gimp-git':
  Gummiboot: Switch to zip archive.