Reland "Update generate_buildbot_json.py and mb.py to use generated .pyl files."

This reverts commit 9b19772dc929059f9d5acb055da56290ab03c989.

Reason for revert: Downstream uses have been updated to reference the
generated gn_isolate_map.pyl.

Original change's description:
> Revert "Update generate_buildbot_json.py and mb.py to use generated .pyl files."
>
> This reverts commit 070fced13afbb20576d1a16835c45d99f976aa55.
>
> Reason for revert: Android builders are failing with
> gn_isolate_map.pyl being removed, e.g.
> https://ci.chromium.org/ui/p/chrome/builders/try/android-internal-rel/21254
>
> Original change's description:
> > Update generate_buildbot_json.py and mb.py to use generated .pyl files.
> >
> > It's no longer necessary to have all of the .pyl files present in
> > //testing/buildbot:
> >
> > * The recipes have been updated to use
> >   //infra/config/generated/testing/gn_isolate_map.pyl. The file has been
> >   present for over a year, so there's no concerns with pinpoint building
> >   revisions that don't contain it.
> > * The json generation script in the angle repo has been updated to only
> >   read the mixins.pyl file from //testing/buildbot since it was only
> >   reusing mixin definitions.
> >
> > This updates //testing/buildbot/generate_buildbot_json.py and
> > //tools/mb/mb.py to read the .pyl files from their generated location in
> > //infra/config/generated/testing, removing the need to sync most of
> > them. The mixins.pyl file still needs to be synced so that it is
> > available to the angle repo's script.
> >
> > Bug: 360187776
> > Change-Id: I4acd677a2c0872a2087270369c1374191f58d650
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5800911
> > Reviewed-by: Ben Pastene <bpastene@chromium.org>
> > Auto-Submit: Garrett Beaty <gbeaty@google.com>
> > Commit-Queue: Garrett Beaty <gbeaty@google.com>
> > Cr-Commit-Position: refs/heads/main@{#1344958}
>
> Bug: 360187776
> Change-Id: I3fa1948be33bce0d9eeef9c787c14ca4c06534da
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5804839
> Auto-Submit: Garrett Beaty <gbeaty@google.com>
> Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#1345008}

Bug: 360187776
Change-Id: Ibb2c0a0dd935e19c40a784d6334b0cba117acdfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5824312
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@google.com>
Cr-Commit-Position: refs/heads/main@{#1348182}