Run next-rails specs if PR comes from next-rails

If the source branch of you PR contains the string next-rails,
we run the spec for the next rails version.

.github/workflows/spec.yml

@@ -2,9 +2,11 @@ name: Specs
 
 on:
   push:
-    branches: [ master ]
+    branches:
+      - master
   pull_request:
-    branches: [ master ]
+    branches:
+      - master
   workflow_dispatch:
 
 jobs:
@@ -30,6 +32,11 @@ jobs:
         suite: [models, features, controllers, ability, leftovers]
     steps:
       - uses: actions/checkout@v2
+      - name: Sets env vars for next-rails
+        run: |
+          echo "BUNDLE_GEMFILE=Gemfile.next" >> $GITHUB_ENV
+          echo "BUNDLE_CACHE_PATH=vendor/cache.next" >> $GITHUB_ENV
+        if: contains(github.head_ref, 'next-rails')
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: 2.5