Fix workflow branch syntax (#90)

In a previous PR we used `branch` syntax to trigger the pipeline when a push on `main` was made. This was a mistake and `branches` is the correct syntax that achieves this.

.github/workflows/lint.yml

@@ -1,6 +1,6 @@
 on:
   push:
-    branch: [main]
+    branches: [main]
   pull_request:
 name: Lint
 

.github/workflows/test.yml

@@ -1,6 +1,6 @@
 on:
   push:
-    branch: [main]
+    branches: [main]
   pull_request:
 name: Run acceptance tests