main.star: use id dimension to select Plan 9 builders

The current Plan 9 builders are expected to be Linux AMD64 hosts in the
shared-workers pool that will provide functional go_plan9_{goarch}_exec
scripts in order to test the Plan 9 Go ports.

For a lack of a better alternative available now, rely on their unique
bot id dimension to tell them apart from other Linux AMD64 hosts in the
shared-workers pool.

For golang/go#62025.

Change-Id: Ib53de20dd4a56dd222b281abc8b7dbbeeb18a0ab
Reviewed-on: https://go-review.googlesource.com/c/build/+/588756
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: David du Colombier <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>