DEV: add stable to github workflows

.github/workflows/plugin-linting.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - master
       - main
+      - stable
   pull_request:
   schedule:
     - cron: '0 0 * * *'

.github/workflows/plugin-tests.yml

@@ -3,6 +3,7 @@ name: Plugin Tests
 on:
   push:
     branches:
+      - stable
       - master
       - main
   pull_request:
@@ -51,6 +52,7 @@ jobs:
       - uses: actions/checkout@v2
         with:
           repository: discourse/discourse
+          ref: "${{ (github.base_ref || github.ref) }}"
           fetch-depth: 1
 
       - name: Fetch Repo Name