The idea is to have 2 runners per machine, since a lot of time is spent building
stage3 and stage4, both of which are largely single-core affairs. This will make
the test steps take longer, however, so the timeouts have been bumped a bit, and
max RSS for the test step has been lowered from 64G to 32G to prevent OOM.
Finally, we now only run a single ReleaseSafe job on PRs; Debug and Release jobs
are limited to pushes.