CI: pin ci/gate, gate and mg-reuse-self-test to X64 runners #6200

The three required jobs use [self-hosted, Linux, clean-room] with no arch label, so they can land on gx10 aarch64 runners that die at job start (0 steps; runs 40003587214 mg-reuse-self-test on gx10-pool4, ci / gate on gx10-build). Add X64 to their runs-on. Revert = one revert PR if the x64 queue exceeds 30 min.