ci: trigger on push to main (#421)

Modifies CI trigger to run when changes are made to the main branch.

Author: tdstein

.github/workflows/ci.yaml

@@ -1,6 +1,9 @@
 name: CI
 on:
-  - pull_request
+  pull_request:
+  push:
+    branches:
+      - main
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true