nixos/gitolite: copy hooks with force

This makes the commonHook option work also for (read-only) Nix store
paths. Currently it fails on the second activation, because the
destination is read-only.

nixos/modules/services/misc/gitolite.nix

@@ -207,7 +207,7 @@ in
             gitolite setup -pk ${pubkeyFile}
           fi
           if [ -n "${hooks}" ]; then
-            cp ${hooks} .gitolite/hooks/common/
+            cp -f ${hooks} .gitolite/hooks/common/
             chmod +x .gitolite/hooks/common/*
           fi
           gitolite setup # Upgrade if needed