x/build: linux-amd64-race is taking about 5 minutes too long for pre-submit purposes

[dmitshur]

The trend of the linux-amd64-race builder taking approximately 20 minutes is fairly visible here, under the Run Duration column:

https://ci.chromium.org/ui/p/golang/builders/try/gotip-linux-amd64-race

We should try to get it down to 15 minutes per #17104. This is the tracking issue for that.

[dmitshur]

Some differences in configuration between the LUCI configuration vs the old coordinator linux-amd64-race trybot:

• the coordinator one has 1 or 2 more test shards
• coordinator skipped GOROOT/test and cmd/internal/bootstrap_test tests (https://cs.opensource.google/go/x/build/+/master:dashboard/builders.go;l=1790;drc=933b1e064385f0c8f70d1dac680468be112970e2)
• coordinator used the slightly newer gen n2-standard-16 machine type (LUCI's linux-amd64-race test shards use n1-standard-16)

@mknyszek also spotted that the work distribution between shards has room for improvement. E.g., in https://ci.chromium.org/b/8755538647004651009, the 4 shards took 4m, 8m, 13min (!), and 9m. (Issue #65814 is related to this.)