uqm: Clean up package expression style

The only functional change here is that I've dropped unzip, because that
dependency doesn't seem to be necessary at all. Despite the broken build
state I've tried building and running The Ur-Quan Masters with an older
nixpkgs revision and it works fine.

Other than that there are no references in the source code for unzip
except in the binary installer and some of the INSTALL files.

The reason why I'm removing the "with pkgs.lib;" is that it makes it
easier to quickly check for errors with "nix-instantiate --parse".

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @jcumming