Use new CI runners

[cgarciae]

Use larger test runners provided by Google. Figure out how to config build.yaml to use them.
@levskaya says:

I think it's a one-line change
something like "runs on" directive
it's in the docs somewhere but I've forgotten the line

[cgarciae]

Additionally, run pytype tests in parallel via bash sub-processes to leverage the additional cores.

[marcvanzee]

Fixed by #2635