[infra] Tighten the path regexp for builder-config-verifier.

builder-config-verifier is currently being triggered for all changes in
//infra/config/generated/builders, which made sense when the builder was
first added and there was only properties.json files in the individual
builder directories. Now however, there are gn-args.json files in the
individual builder directories and gn_args_location.json in the root
builders directory and targets spec files will be added in the near
future. With the current path regexp, changes that don't modify the
properties.json files still launch builder-config-verifier, which has
nothing to verify. This changes it so that only changes to the
properties.json files will trigger builder-config-verifier.

Change-Id: I8a645ab52f1a266213d1e20a23bca378c8cb7601
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5089849
Commit-Queue: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Cr-Commit-Position: refs/heads/main@{#1233515}