Review shell fails when there are single-file inputs.

[corngood]

Some things that get covered by nixpkgs-review install with something like touch $out, which can result in this:

       > error: The store path /nix/store/b5q8bljrad33lmmypn4dd2firrjg1rs5-equal-contents-builddotnetmodule-sets-assemblycopyrightattribute-with-structured-attributes is a file and can't be merged into an environment using pkgs.buildEnv! at /nix/store/77my1q03gyb36mhs0sij87yr710w9gha-builder.pl line 122.

I've fixed a couple by changing them to mkdir $out, but occasionally there's a reason to have content in the output file, so perhaps this should be supported?