Remove FORCE_USE_SYSTEM_CLANG in favour of always using the system Clang and lld for macOS cross-compilation; rather than downloading precompiled blobs.

Backport of core#30201 and core#30231.

Depends on D17265.

Note the CI update will be done in another diff. It only affects the cross osx build on CI which is currently paused.

Run the guix osx build.