This is a workaround for NixOS/nix#4295, which caused single-user Linux
Nix installations using sandboxed builds to start failing to build
fetchzip derivations after 4a5c49363a.
In short: removing write permissions for the entire directory is great,
except we then can't rename(2) it to the final Nix store path out of the
sandbox, because we don't have write permission on the directory and
thus cannot update the ".." directory entry.
1f6d750097