lib: Define an alias OSTREE_REPO_MODE_ARCHIVE

lib: Define an alias OSTREE_REPO_MODE_ARCHIVE

For the old `OSTREE_REPO_MODE_ARCHIVE_Z2`.  Use it mostly tree
wide except for the repo finder tests (to avoid conflicting with
some outstanding PRs).

Just noted another user coming in some of those tests and wanted to do a
cleanup.

Closes: #1209
Approved by: jlebon