main.star: stop skipping golang.org/x repos on linux-s390x builder

The builder is running in the main Go repo. Remove the temporary skip
of golang.org/x repos as the next step of its onboarding.

Also move it to LOW_CAPACITY_HOSTS since by now its capacity is known
to be fixed.

For golang/go#67307.

Change-Id: Ib42d9016ba2d042bdcf87cf38544e401abbfe805
Reviewed-on: https://go-review.googlesource.com/c/build/+/671958
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur