Currently when you repeatedly create a bootstrap mirror that includes
`clingo-bootstrap@spack` you get different tarballs every time.
This is a general problem with mirroring checkouts from version control
as tarballs. I think it's best to create tarballs ourselves, since that way we
have more control over its contents.
This PR ensures normalized tarballs like we do for build caches:
- normalize file permissions (in fact that was already inspired by git, so
should be good)
- normalized file creation/modification time (timestamp 0)
- uid / guid = 0, no usernames
- normalized gzip header
- dir entries are ordered by `(is_dir, name)` where strings are not locale aware ;)
- POSIX says st_mode of symlinks is unspecified, so work around it and
force mode to `0o755`
28675478ce