devel/qt6-tools: Enable build with ccache after [1]

We don't have to build with a particular llvm version, but need to
specificy an llvm version to link against. This restores build behavior
before [1].

[1] https://cgit.freebsd.org/ports/commit/?id=4df0e66a3662ac9d427c2dcd79eb8555236083dd