fix: reduce redundancy in workflow jobs

This fixes CI so it doesn't run all the jobs for "push" and
"pull_request" for PRs. It also will cancel any in-progress jobs if I
push a new commit.

Author: willkg

.github/workflows/test.yml

@@ -1,7 +1,14 @@
 ---
 name: Test
 
-on: [push, pull_request]
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   build: