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.

pkgs/build-support/src-only/default.nix

···
 stdenv.mkDerivation (args // {
   name = "${name}-source";
   installPhase = "cp -r . $out";
+  outputs = [ "out" ];
+  separateDebugInfo = false;
   phases = ["unpackPhase" "patchPhase" "installPhase"];
 })