[CI] Build the fuzzer on diffs that touch the fuzzer files

Summary:
This will help catching fuzzer build issues early, and give confidence
to reviewers. It is incomplete because other source files can impact the
fuzzer output but this is better than nothing.

Test Plan: This can't be tested until it lands.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8943