ci chore: explore CI triggering

[Lee Read]

Currently

We've moved to trigger tests on all: push, pull_request.

This is good because CI tests will be run on all pushes which will allow contributors to verify their work in their forks/branches before creating a PR.

But

A downside is that CI tests are triggered twice on PR pushes.

This is not a big deal for some libs, but an annoyance for Etaoin whose tests take a while to run and are, frankly, flakey.

My current hope is that telling GitHub to not trigger on pull_request synchronize, will do the trick.

While I'm at it

We use git tags to tag a release version (and soon to trigger a release #529).

I don't think the tests workflow should be triggered solely by a git tag ever.

My hope is that we can tell GitHub to not trigger tests on any tag via:
on: push: tags_ignore: "*" (yaml syntax to be corrected).