Backport of CI: run tests on pull_request and push to main/release into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #15690 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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Running tests on: push prevents GitHub from showing the workflow approval button, which prevents tests from being run on community-contributed (or even just non-Nomad HashiCorp folks) PRs. Running on: pull_request automatically picks up opened, reopened, and synchronize hooks (where "synchronize" means a push to HEAD on the PR's branch, so that'll pick up rebases and updates).

But we also want to run tests on main and the various release backport
branches, so retain a on: push for those.

Closes #15632

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[tgross]

Closing because we don't have test-core in release/1.2.x

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