Copy Wasmtime's filters for running CI builds

.github/workflows/main.yml

@@ -1,5 +1,19 @@
 name: CI
-on: [push, pull_request]
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+defaults:
+  run:
+    shell: bash
+
+# Cancel any in-flight jobs for the same PR/branch so there's only one active
+# at a time
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   test: