LTO links lose the jobserver

01c35e6 says:

Almost nothing sccache spawns can use a jobserver. It reaches a child through CARGO_MAKEFLAGS, and only rustc reads it: preprocessors, version probes and C/C++ compilers all ignore it. So stop sharing by default and let the callers that spawn rustc ask, via RunCommand::share_jobserver.

But clang/gcc can use the jobserver when running an LTO link. We should continue to pass the jobserver for those jobs.