Show PR experiment info in Cloud Build Page.

[DonggeLiu]

Currently, we have to write down the experiment report and data info manually. This is annoying because the experiment trigger always parses the last comment, so we must wait until the experiment has launched before pasting the links.

This PoC shows a way to automate this, which prints the PR ID as an example. We can extend the code to show the experiment report and data.

[DonggeLiu]

Some extensions after the task is done:

1. Check if the PR modified gcb_run_experiment.py and remind the owner.
2. Only show the correct experiment report URL based on the fuzzer.
3. Filter outputs. Omit everything from step 2 because PR experiment does not change that code section. Only show the customized log in step 1 and the experiment launch status in step 3.
4. Also show the experiment info in a GitHub comment.