buildstream/_artifactcache/cascache.py: Set 0644 rights to pulled files

This was broken by 9252a181.

buildstream/_artifactcache/cascache.py

@@ -854,6 +854,7 @@ class CASCache(ArtifactCache):
         with tempfile.NamedTemporaryFile(dir=self.tmpdir) as f:
             self._fetch_blob(remote, digest, f)
 
+            os.chmod(f.name, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
             added_digest = self.add_object(path=f.name, link_directly=True)
             assert added_digest.hash == digest.hash