Don't duplicate CI checks on PRs

By default the `pull_request` trigger will run on every push to the PR
branch.

So we are wasting CI minutes and electricity by running our CI checks
twice on every push to a PR branch.

Instead, this makes checks run 1x per PR, and then also run on every
merge to `master`, to ensure that `master` stays green. This latter
check is normally useless, but occasionally if there's drift of some
kind between when CI runs on a PR and when it's merged, then this can
help identify the issue.

A more common pattern is simply to only run on PR's, but given we
haven't previously been enforcing "only merge via PR"
(#1073) I thought might
be best to keep checking `master` as well until that's changed.

The one thing we stop doing with this change is checking on push to
branches that aren't PR branches... ie, if a maintainer is working on
testing something. But they may not even care about running CI on this
branch, and if they do, it's easy to run the tests locally, or open a
draft PR...So I don't see the point of preserving that behavior.

.github/workflows/check.yml

@@ -1,8 +1,10 @@
 name: Check
 
 on:
-  - push
-  - pull_request
+  push:
+    branches:
+      - "master"
+  pull_request:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}

.github/workflows/test.yml

@@ -1,8 +1,10 @@
 name: Test
 
 on:
-  - push
-  - pull_request
+  push:
+    branches:
+      - "master"
+  pull_request:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}