cmdlib: Ensure bare-user tmpdir is in $workdir/tmp

Linux `tmpfs` still doesn't support `user.` xattrs, and so for
toolbox-like containers that use a (proper!) `tmpfs` for `/tmp`
instead of having it be `overlayfs`, we need to ensure that
we create our tempdir in `$workdir/tmp` - the workdir must have
`user.` xattr support for the `cache/repo-build` anyways.

Closes: #2904
(cherry picked from commit e024805)

dustymabe: This cherry pick needed manual merge conflict resolution.
           The function call for import_ostree_commit() changed in
           816ebae to pass the workdir in. I evaluated the changes
           in e024805 and 816ebae and determined this to be the most
           surgical change.

src/cosalib/cmdlib.py

@@ -277,7 +277,8 @@ def import_ostree_commit(repo, buildpath, buildmeta, force=False):
             gid = os.getgid()
             subprocess.check_call(['sudo', 'chown', '-hR', f"{uid}:{gid}", repo])
         else:
-            with tempfile.TemporaryDirectory() as tmpd:
+            tmpdir = os.path.join(os.getcwd(), 'tmp')
+            with tempfile.TemporaryDirectory(dir=tmpdir) as tmpd:
                 subprocess.check_call(['ostree', 'init', '--repo', tmpd, '--mode=bare-user'])
                 subprocess.check_call(['rpm-ostree', 'ex-container', 'import', '--repo', tmpd,
                                        '--write-ref', buildmeta['buildid'], 'ostree-unverified-image:oci-archive:' + tarfile])