[infra][build] Set HOME=/root on GCB when doing fuzzer builds.
GCB passes HOME as env var to the docker container. It sets
HOME to /builder/home which is persisted accross builds.
This issue causes build breakages in
https://github.com/google/oss-fuzz/issues/6035
and possibly https://github.com/google/oss-fuzz/issues/5317.
Perhaps more insidiuosly it can cause fuzzers to be built with
the wrong instrumentation.
The OSS-Fuzz builder and runner images have libunwind. Since
the OSS-Fuzz runner images aren't actually used to run fuzzers on
ClusterFuzz and are just actually used for testing(!) this passed
CI before even though it failed immediately on ClusterFuzz.
I've tested that with this fix, the fuzzer works on ClusterFuzz.
jonathanmetzman