main.star: add playground repo

The previous infrastructure did not display x/playground on the
post-submit dashboard, but they did offer trybots. LUCI trybots
are required now, so it's a good time to add basic coverage for
x/playground. Being able to run all of its tests that have more
dependencies is for later.

For golang/go#24823.

Change-Id: I550aed2f563aa31a061e4646538853e18ebd2fc9
Reviewed-on: https://go-review.googlesource.com/c/build/+/566417
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

Author: dmitshur