regexp: apply canonicalization to scripts lookup

Fixes #77698.

Change-Id: I5764e987de3dc70054f5e4361dbe7d9e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/747020
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>