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.

Changed files
+2
pkgs
build-support
src-only
+2
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"];
})