Blobstore backup uses a separate tempdir
Use single quotes for uninterpolated strings
Prefer single quotes, %W and implicit subject
Assume that tar is on the director's PATH

We confirmed that this is true on an actual deployment
Use single quotes for uninterpolated strings
Refactoring of tar_gzipper to use a base_dir and relative paths.
Tarballs now have pretty paths.

[Finishes #52425805]