Don't run tests after a PR is merged

[shralex]

Currently, once Github tests pass and the PR merges in our Google repo, our workflow merges the PR in Github. Then, after the merge, tests are again triggered in Github on the main branch. We already have regular scheduled tests on main, so this isn't needed.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.

[richjames0]

lgtm