Configure Kokoro to do podcvd E2E tests

[0405ysj]

I originally implemented to trigger podcvd E2E tests from GitHub action, but some limitations were observed.

• (Critical reason) A lot of test cases weren't able to fetch required artifacts. For example, when the branch is git_main.
• GitHub runner has very limited computing resources. I'm expecting that as the root cause of current flakiness of bugreport test. (90 seconds locally, about 300 seconds on GitHub runner)
• Looking forward to make test cases non-exclusive on Kokoro-based podcvd testing, as podcvd can deal with concurrent instance creation.

So I suggest to move podcvd E2E test into Kokoro. GitHub action should be kept during transition period for keeping podcvd stable, and will be cleaned up afterwards.

Context: b/527644057

[0405ysj]

Marked this PR as draft for now, as yet I cannot trigger Kokoro for podcvd testing.

[0405ysj]

Marking as draft for a moment as it seems tools/buildutils/installbazel.sh is broken regardless of this PR.