5 shards, each of which spins up NumCPU processes,
each of which is running at GOMAXPROCS=NumCPU,
is too much for one machine. It makes my laptop unusable.
It might also be in part responsible for test flakes
that require a moderately responsive system,
like #18589 (backedge scheduling) and #19276 (locklinear).
It's possible that Go should be a better neighbor in general;
that's #17969. In the meantime, fix this corner of the world.
Builders snapshot the world and run shards on different
machines, so keeping sharding high for them is good.