ClangBrew, ClangNative and GccNative now export SPC_DEFAULT_RANLIB
alongside SPC_DEFAULT_AR. UnixCMakeExecutor honours both when
generating the Linux toolchain file, so cmake uses the toolchain's
ar/ranlib (e.g. zig-ar/zig-ranlib for archives that the system
ranlib does not understand) instead of /usr/bin/ranlib.