main.star: don't hide openbsd-amd64_7.6 from presubmit

Otherwise it's not possible to test that a fix (or skip) for the known
issue works, as I ran into in CL 658015:

	Failed to compute tryjob requirement. Reason:

	builder "golang/try/x_tools-gotip-openbsd-amd64_7.6" is included but not defined in the LUCI project

For golang/go#72145.

Change-Id: I4e447a8dadf2ad3a1def6b9caab9140bf08b61c4
Reviewed-on: https://go-review.googlesource.com/c/build/+/658016
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: dmitshur