main.star: enable openbsd-amd64_7.6 for x/ repos

The image was built successfully (on second try) and has proven to work
in the main Go repo (https://ci.chromium.org/b/8721307255879001089), so
take the next step of enabling this builder for golang.org/x repos.

For golang/go#69823.

Change-Id: Ic5c8aee36f3d2f0f3ff57525f996a3da600c25a0
Reviewed-on: https://go-review.googlesource.com/c/build/+/654675
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Junyang Shao <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur