infra: Register 'chromium/fuzz' as a trybot-CI mirroring recipe

With the work in https://crbug.com/343453596, trybots running the
chromium/fuzz recipe now support and use configs from CI bots that
are configured to be their mirrors. This means they're now present
in the starlark builder graph that maps what builders mirror what.

This is the same graph that's crawled to determine what bots to put on
the mega CQ. Which means we can now allowlist the fuzz recipe for that
purpose, which puts all fuzz CI bots with try mirrors on the mega CQ.

Bug: 343453596
Change-Id: I9845da63c8ed260ef67d92a5e7c1f0e3ba839eb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5599934
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Garrett Beaty <gbeaty@google.com>
Cr-Commit-Position: refs/heads/main@{#1310806}