Update mtime of libclang_rt.osx.a after moving (#61647)

`mv` preserves the mtime of `libclang_rt.osx.a`, which is older than the
mtime of `$(build_prefix)/manifest/compilerrt`, causing subsequent
builds to fail with `mv: cannot stat 'usr/lib/darwin/libclang_rt.osx.a':
No such file or directory`. Fix this by touching `libclang_rt.osx.a`
after installing it.