[CI-only] Update build trigger

[mdeggies]

This update will trigger a build of nomad in GitHub Actions on every push to a branch, except docs or backport/docs branches. This allow/denylist can be modified further if needed based on the team's needs, but is meant to help us catch build issues earlier than at merge-time, and enable faster testing of changes/feature sets. The main con to this approach is that builds take ~20 minutes on average, and circleci tests in the repos are taking about ~15 mins, so an additional ~5 mins (give or take) is added to the feedback loop. We discussed this here and the team seemed 👍 that the pros outweigh the cons here.

[tgross]

Leaving a note that there's still some open discussion on this one internally: https://hashicorp.slack.com/archives/CUYKT2A73/p1663623216766039

[mdeggies]

I updated this PR to always trigger builds in main and the release/** branches. This will avoid backport issues in the build workflow (from main into the release branches) and let us avoid having to manually allowlist new release branches in the build workflow. This also matches what is in the release configuration file: https://github.com/hashicorp/nomad/blob/main/.release/ci.hcl#L14-L17.

[github-actions[bot]]

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