[libcxx] CI: only build native target for bootstrapping-build

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D113950

GitOrigin-RevId: 35f798d05d5138613e1392ec1630eec93b0caff9
1 file changed