main.star: mark x/oscar as a PT.TOOLS repository

Add x/oscar as a 'TOOLS' repository.

Change-Id: Ie35d9b63706bc9fcdab3a85cfffd384c613fa2b4
Reviewed-on: https://go-review.googlesource.com/c/build/+/596675
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Than McIntosh <[email protected]>

Author: thanm