fetchgit: chmod -w breaks subsequent mv

```
/nix/store/rc94la2h0pwkshbgmd7kz5nhklkdqyr2-coreutils-8.21/bin/mv: cannot move ‘/tank/proger/.cache/fetchgit/archives/nixpkgs/true/c8be814f254311ca454844bdd34fd7206e801399/nixpkgs.WPcG9d’ to ‘/tank/proger/.cache/fetchgit/archives/nixpkgs/true/c8be814f254311ca454844bdd34fd7206e801399/nixpkgs’: Permission denied
/nix/store/rc94la2h0pwkshbgmd7kz5nhklkdqyr2-coreutils-8.21/bin/mv: cannot move ‘/tank/proger/.cache/fetchgit/archives/nixpkgs/true/499c51016ef67bd0b158528dbff17ed6ecedd78b/nixpkgs.JV6O8G’ to ‘/tank/proger/.cache/fetchgit/archives/nixpkgs/true/499c51016ef67bd0b158528dbff17ed6ecedd78b/nixpkgs’: Permission denied
```