Execute build and e2e tests on push, not pr.

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As configured, the same set of tests runs on pull request and on the merge queue. This is redundant, and flakes can cause large delays. Ideally, we should reduce the redundancy, either by pushing tests from pull request to merge, or vice versa. The consensus is on the former, to ensure that the code at HEAD is most likely to be passing tests.

As I understand it, the push GitHub workflow event triggers on the merge queue, so the new pull request workflow triggers on the pull_request event. This workflow does not run any of the build or end-to-end checks, which can take a long time, only the static and unit checks.