Run linting and testing GitHub workflows on PRs

Currently, code is not automatically linted and tests are not
automatically run on pull requests. This happens because the workflows
which would perform these steps are not configured to run on pull
requests but only on a push to `main`. This PR modifies the workflows to
run on the `pull_request` event.

.github/workflows/shellcheck.yml

@@ -3,7 +3,10 @@ name: shellcheck
 permissions:
   checks: write
 
-on: [push]
+on:
+  push:
+    branches: [main]
+  pull_request:
 
 jobs:
   lint:

.github/workflows/test-monorepo.yml

@@ -1,5 +1,8 @@
 name: test-monorepo
-on: [push]
+on:
+  push:
+    branches: [main]
+  pull_request:
 jobs:
   # test npm publish dry-run
   checkout_publish_skunkworks_dry_run:

.github/workflows/test-polyrepo.yml

@@ -1,5 +1,8 @@
 name: test-polyrepo
-on: [push]
+on:
+  push:
+    branches: [main]
+  pull_request:
 jobs:
   # test npm publish dry-run
   checkout_publish_controllers_dry_run:

.github/workflows/test.yml

@@ -1,5 +1,8 @@
 name: test
-on: [push]
+on:
+  push:
+    branches: [main]
+  pull_request:
 jobs:
   test:
     runs-on: ubuntu-20.04