fix: run the presubmit pipeline for PRs

commit 5eabe549e4f7fc98bbdf4e7b285cec00e5da4e99 refactored the workflows
so that they were orchestrated with reusable workflows. an unintended
side-effect of the rules that were created prevents the presubmit
pipeline from executing for pull requests made against this repository.

this change refactors this, so that the presubmit pipeline will _only_
run for pull requests against the default tree of this repository. we
explicitly remove the `on: push` event for it, in order to avoid overlap
that will occur for commits made in this repository that are also part
of a PR.

Change-Id: I56abc35c98cc40a06066952f2b6ed9194b1ab903