[fuzzcoverage] Add `builderless` tag so that linux-fuzz-coverage knows which
machines to target.

We committed the linux-fuzz-coverage builder, but it expected machines
allocated to it specifically, rather than using the machines we added to
the builderless pool in cl/5397682.

Therefore the builder right now has no machines it can use.

Update the builder to target the buidlerless pool in a manner similar to
the code coverage bots.

Bug: 1356583
Change-Id: I911b50462a48ee553bd55db77fc55c67af2b3c2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4285397
Commit-Queue: Julia Hansbrough <flowerhack@google.com>
Reviewed-by: Prakhar Asthana <pasthana@google.com>
Cr-Commit-Position: refs/heads/main@{#1108705}