libfetchers/path: set `lastModified` to path's mtime

When importing e.g. a local `nixpkgs` in a flake to test a change like

    {
      inputs.nixpkgs.url = path:/home/ma27/Projects/nixpkgs;
      outputs = /* ... */
    }

then the input is missing a `lastModified`-field that's e.g. used in
`nixpkgs.lib.nixosSystem`. Due to the missing `lastMoified`-field, the
mtime is set to 19700101:

    result -> /nix/store/b7dg1lmmsill2rsgyv2w7b6cnmixkvc1-nixos-system-nixos-22.05.19700101.dirty

With this change, the `path`-fetcher now sets a `lastModified` attribute
to the `mtime` just like it's the case in the `tarball`-fetcher already.
When building NixOS systems with `nixpkgs` being a `path`-input and this
patch, the output-path now looks like this:

    result -> /nix/store/ld2qf9c1s98dxmiwcaq5vn9k5ylzrm1s-nixos-system-nixos-22.05.20220217.dirty

0 files changed, 0 insertions, 0 deletions