The fuzzers build httplib.h and the targets under test/fuzzing, so a pull
request that touches neither has nothing for CIFuzz to exercise. Fuzzing
is by far the longest job in CI: 600 seconds of fuzzing on top of
building the OSS-Fuzz image, which came to 12m20s on a recent run while
every other job finished within 5m7s.
Filter the trigger by path rather than shortening fuzz-seconds. OSS-Fuzz
recommends 600 seconds as a minimum, and the budget is divided among all
of the project's fuzz targets, so with five targets a shorter run would
leave each one well under two minutes. Skipping the job for
documentation-only changes cuts the wait without giving up any fuzzing
on the pull requests that do reach the parsers.
