Fix flaky Vivado failures (perhaps use fewer cores)

[gussmith23]

Vivado uses a ton of cores, so I suspect we're getting some weird error after we turned up the number of parallel jobs. We just need to tune down the number of cores.

Errors on:

baseline_synthesis:vivado_neq16_2
baseline_synthesis:vivado_eq1_2

I can't replicate when running in single-job mode, but last time I replicated it seemed like some very odd system-level error about not being able to load some TCL library.

[gussmith23]

Lowered the number of parallel jobs in 18b81315533f12aed7baa5d5b5dd8c6a134c0dc2, we'll see if that fixes.

[gussmith23]

These have gone away. Must be the number of parallel jobs. It may be time to switch to a staged evaluation, where we do the finicky stuff like Vivado first...because we're wasting a lot of time not using all possible cores!