x/build: shard `linux-amd64-longtest-race` and `windows-amd64-longtest-newcc` SlowBots

[bcmills]

In #37439 we set numTryTestHelpers for the -longtest builders in order to speed them up when run as SlowBots.

Since that time, we have added two new -longtest builders: windows-amd64-longtest-newcc and linux-amd64-longtest-race. Those should also be sharded, for the same reason.

[heschi]

cc @thanm @mknyszek since they added the two builders. @cherrymui, can you take a look as on duty?

[gopherbot]

Change https://go.dev/cl/452857 mentions this issue: dashboard: shard linux-amd64-longtest-race and windows-amd64-longtest-newcc builders