Upstreams: don't write metadata directory to upstream DB (#15526)

When trying to use an upstream Spack repository, as of f2aca86 Spack
was attempting to write to the upstream DB based on a new metadata
directory added in that commit. Upstream DBs are read-only, so this
should not occur.

This adds a check to prevent Spack from writing to the upstream DB

lib/spack/spack/database.py

@@ -311,7 +311,7 @@ def __init__(self, root, db_dir=None, upstream_dbs=None,
         if not os.path.exists(self._db_dir):
             mkdirp(self._db_dir)
 
-        if not os.path.exists(self._failure_dir):
+        if not os.path.exists(self._failure_dir) and not is_upstream:
             mkdirp(self._failure_dir)
 
         self.is_upstream = is_upstream