Github Actions: Run tests on all PRs (#3709)

Chained PRs (like https://github.com/plausible/analytics/pull/3708)
don't currently run tests. This PR fixes that.

.github/workflows/build-private-images.yml

@@ -5,7 +5,6 @@ on:
     branches: [master, stable]
     tags: ['r*']
   pull_request:
-    branches: [master]
     types: [synchronize, labeled]
 
 concurrency:

.github/workflows/elixir.yml

@@ -4,7 +4,6 @@ on:
   push:
     branches: [master, stable]
   pull_request:
-    branches: [master, stable]
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}

.github/workflows/node.yml

@@ -4,7 +4,6 @@ on:
   push:
     branches: [master, stable]
   pull_request:
-    branches: [master, stable]
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}

.github/workflows/terraform-e2e.yml

@@ -7,8 +7,6 @@ on:
     paths:
       - 'test/e2e/**.tf'
   pull_request:
-    branches:
-      - master
     paths:
       - 'test/e2e/**.tf'