main.star: add a linux-amd64-racegocommand-noopt builder at tip

Add a post-submit-only builder that runs at tip with a new racegocommand
mod, one which will reinstall the go command with race mode enabled.
Also try combining it with a noopt mod, since the existing noopt builder
seems to be one where these races are triggered more frequently.

For golang/go#79779.

Change-Id: I60eebf686e6ccbc7407980db8ca7c1560f79806c
Reviewed-on: https://go-review.googlesource.com/c/build/+/785920
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>