Reduce number of github runners our e2e tests use in PRs

[chatton]

Summary of Bug

Because we spin up a lot of e2e tests, we can inadvertently slow down CI for other projects within the org. We can consider some options to reduce usage of the hosts.

Some options:

• Run smaller subset of tests on PRs
• Run E2Es only prior to merge when ready for review.

If we reduce PR e2es, we should make sure to run all E2Es nightly.

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[chatton]

@tac0turtle please leave any more suggestions you have

[DimitrisJim]

How relevant is this now that we've been added to paid runners (context in the sdk channel on slack). Do this and issue #3397 need to be closed? Should we talk about it in the next sdk call?