[CIFuzz]: CIFuzz seems to be incompatible with forks #3731
Description
I opened a PR against a fork and CIFuzz failed there with
2020-04-29 05:10:21,600 - root - DEBUG - Stderr of command 'git clone https://github.com/systemd/systemd systemd' is Cloning into 'systemd'...
.
2020-04-29 05:10:21,845 - root - DEBUG - Stderr of command 'git fetch origin refs/pull/1/merge' is fatal: Couldn't find remote ref refs/pull/1/merge
.
Running: docker build -t gcr.io/oss-fuzz/systemd projects/systemd
2020-04-29 05:10:21,846 - root - ERROR - Can not check out requested state.
2020-04-29 05:10:21,846 - root - ERROR - Error building fuzzers for project systemd with pull request refs/pull/1/merge.
I'm not sure what to do about it but given that GitHub Actions are turned on by default everywhere I'd say it should probably be skipped if the target repository is different from what CIFuzz expects. On Travis CI something like [[ "$TRAVIS_REPO_SLUG" = "systemd/systemd" ]] is used to protect the scripts from forks.