Mirrors: perform checksum of fetched sources

Since cache_mirror does the fetch itself, it also needs to do the
checksum itself if it wants to verify that the source stored in the
mirror is valid. Note that this isn't strictly required because fetching
(including from mirrors) always separately verifies the checksum.

lib/spack/spack/stage.py

@@ -500,6 +500,7 @@ def cache_mirror(self, stats):
             stats.already_existed(absolute_storage_path)
         else:
             self.fetch()
+            self.check()
             spack.caches.mirror_cache.store(
                 self.fetcher, self.mirror_paths.storage_path)
             stats.added(absolute_storage_path)