Do not run duplicate CI checks

[alexevanczuk]

Previously, I merged https://github.com/Shopify/packwerk/pull/236 (has screenshots), since we see duplicate checks run on every build.

This had to be reverted here https://github.com/Shopify/packwerk/pull/240 because it ended up not running checks on push from a fork from an external contributor.

Searching github community, I found a suggested solution: https://github.com/orgs/community/discussions/26940#discussioncomment-5686753

We'll want to test this solution and revert if it doesn't work in the following contexts:

• running checks for a maintainer PR
• running checks for an external contributor PR (from a fork)
• running tests on main
• running tests on a branch that is NOT a PR. Currently I'm confident this will not work – do folks use branches without PRs and expect/need tests to run?

[alexevanczuk]

@gmcgibbon We could add v* as one of the branches that always gets CI, so maintenance versions can have ci with a branch name like v2.x, for example.

Happy to close this if you think the less redundant output is not worth the additional complexity.

[gmcgibbon]

Let's add *-stable to the list because that's the naming scheme we use. https://github.com/Shopify/packwerk/tree/2-stable. I don't object to trying this, so I'll approve once that's added.

[alexevanczuk]

Okay done @gannon!