ci: use nextest in CI

[galargh]

Testing

I'm comparing the test runtimes only between https://github.com/filecoin-project/ref-fvm/actions/runs/1848484995 and https://github.com/filecoin-project/ref-fvm/actions/runs/1843583595

test-fvm

0.02s (nextest) vs. 0.01s (test)

test

1.79s (nextest) vs. 9.32s (test)

conformance

244.62s (nextest) vs. 302.89s (test)

[galargh]

This is starting to approach my "newness" budget, but 20% is hard to pass up.

That's a fair point. As for the runtime gains it might be coming with - I'm intending to, at the very least, do a couple more runs to get more data points to make sure it wasn't a one-off. I'll take this PR out of draft when I feel more confident such gains are indeed realistic.