Fix engine fuzzers blobstore downloads

[dylanjew]

#5380 fixed the issue where our queue was getting flooded with bad tasks but broke libfuzzer because engine fuzzers do not need a blobstore_key. Instead it skips the function that was causing an error for the misconfigured fuzzers

I queried the Datastore and confirmed that when builtin=True, the blobstore_key is null for the engine fuzzers: syzkaller, googlefuzztest, centipede, libfuzzer. Additionally, the invalid fuzzer with "Name/ID": "name=ochang_js_fuzzer_test" is also there, which was what caused the initial errors

This fixed the invalid FuzzerJob handling, but caused an issue with other libfuzzer fuzzers because libfuzzer has no blobstore key, but skips this look up that would fail because it has not blobstore key since builtin = False

clusterfuzz/src/clusterfuzz/_internal/bot/tasks/setup.py

Lines 606 to 608 in 05360c5

	if not fuzzer.builtin:
	update_input.fuzzer_download_url = blobs.get_signed_download_url(
	fuzzer.blobstore_key)

[g-ortuno]

How hard would it be to add a test as a follow up? 👀

[dylanjew]

How hard would it be to add a test as a follow up? 👀

Good point. I'll add a test 😄

[dylanjew]

How hard would it be to add a test as a follow up? 👀

Good point. I'll add a test 😄

Done!

[g-ortuno]

lgtm still!

[dylanjew]

Fixed formatting