Make workflow targets more specific (#87)

This ensures that:
- Acceptance tests ar run on Pull requests
- Lint is run on `main`
- The workflows are executed only once on PRs

.github/workflows/lint.yml

@@ -1,4 +1,7 @@
-on: pull_request
+on:
+  push:
+    branch: [main]
+  pull_request:
 name: Lint
 
 jobs:

.github/workflows/test.yml

@@ -1,4 +1,7 @@
-on: push
+on:
+  push:
+    branch: [main]
+  pull_request:
 name: Run acceptance tests
 
 jobs: