srcOnly: fix with separateDebugInfo and/or multiple outputs
Before this change `srcOnly git` gives:
duplicate derivation output 'debug', at pkgs/stdenv/generic/make-derivation.nix:270:7
This was because separateDebugInfo = true was passed on to the srcOnly
mkDerivation as well as the outputs list _including_ the debug output.
Luckily we don't need to untangle this mess since srcOnly is only
supposed to have a single output anyways.