main.star: add freebsd-amd64-race

Move the remaining freebsd-amd64 builder configuration from old infra,
and then stop it there (CL 646515).

For golang/go#61095.
For golang/go#71556.

Change-Id: I5b12e139458f7ee9ff97061d23d27fe47ee9937e
Reviewed-on: https://go-review.googlesource.com/c/build/+/646136
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur