Take a lock before querying installed_dependents (#44301)

Co-authored-by: Harmen Stoppels <me@harmenstoppels.nl>

lib/spack/spack/cmd/uninstall.py

@@ -151,7 +151,8 @@ def is_installed(spec):
         key=lambda s: s.dag_hash(),
     )
 
-    return [spec for spec in specs if is_installed(spec)]
+    with spack.store.STORE.db.read_transaction():
+        return [spec for spec in specs if is_installed(spec)]
 
 
 def dependent_environments(