ci: push trigger only on trunk branch and tags

[c-dilks]

If we are following trunk-based development with PRs, then only the trunk branch (development) should trigger on push.

Resolves #104

Do we not want non-development branches running automatic tests?

[c-dilks]

I can't figure out the "best" way to do this such that everyone will be happy, but I would really like to stop the doubled workflow runs. After testing a bunch, I see 3 options for us to choose from:

1. Trigger on all PRs and all commits (what we currently do)

on:
  pull_request:
  push:

• PRO: people working on "private" branches get CI triggers
• CON: identical workflows run twice for every commit

2. Trigger on all commits always

on:
  push:

• PRO: resolves doubled workflow issue, since no pull_request trigger
• CON: workflow runs have no link back to the PR and instead just link to the commit; this is rather annoying, in my opinion

3. Trigger on PRs, and only on pushes to development

on:
  pull_request:
  push:
    branches: [ 'development' ]

• PRO: resolves doubled workflow issue
• CON: people working on "private" branches do not get workflow runs

After testing, I still prefer option 3. IMO collaborators should learn to always create PRs, whether they intend their changes to be merged or not (it's fine to create a test PR and close it). I would prefer draft PRs and frequent, smaller commits, rather than a sudden giant PR.

Changes should not be merged unless there is a PR, and tests will always run on a PR. If someone wants to wait until they are "done" to make a PR, and the tests suddenly fail, then that is their problem.

[baltzell]

Number 3 it is.

[c-dilks]

Number 3 is implemented, including tag triggers, so this is ready.