Fixes build failure on `staging-next` introduced by commit c817efe6
("gcc: extend stripping of .a libraries and .o objects"):
...-bootstrap-stage3-stdenv-linux/setup: line 106: pushd: no other directory
Build failure happens because `libgccjit` has different `outputs` layout
compared to the rest of `gcc` derivations:
outputs = [ "out" "man" "info" ] ++ lib.optional (!langJit) "lib";
The change carves out `$lib` lib handling part to avoid `gcc` rebuild
in `staging-next`.
1f7186ab5c