Don't set the HOST_JOB_SELECTION env var to a 404 error page (#4690)

#4663 was not widely deployed
but it stops fuzzing wherever HOST_JOB_SELECTION isn't set. That's
because it gets set to a 404 HTML page if unset. This in turn interferes
with job selection.

Author: jonathanmetzman

docker/base/setup_clusterfuzz.sh

@@ -19,8 +19,13 @@ if [ -z "$DEPLOYMENT_BUCKET" ]; then
 fi
 
 if [ -z "$HOST_JOB_SELECTION" ]; then
-  # Get list of jobs to focus on from instance metadata, if any.
-  export HOST_JOB_SELECTION=$(curl -H "Metadata-Flavor: Google" http://metadata.google.internal/computeMetadata/v1/instance/attributes/host-job-selection || true)
+  HOST_JOB_SELECTION=$(curl -sf -H "Metadata-Flavor: Google" http://metadata.google.internal/computeMetadata/v1/instance/attributes/host-job-selection)
+  # Don't set the env var to a 404 error page.
+  if [ $? -eq 0 ]; then
+    export HOST_JOB_SELECTION
+  else
+    unset HOST_JOB_SELECTION
+  fi
 fi
 
 CLUSTERFUZZ_FILE=clusterfuzz_package.zip