Include edge Rails in the build matrix

Highlighting an incompatibility with the new implementation of
`[ActiveSupport::OrderedOptions#dig]`.

[^1]: 5c15b586aa

.github/workflows/ci.yml

@@ -8,12 +8,15 @@ jobs:
           - "2.7"
           - "3.1"
           - "3.2"
+        gemfile:
+          - Gemfile
+          - gemfiles/rails_edge.gemfile
         continue-on-error: [false]
-
     name: ${{ format('Tests (Ruby {0})', matrix.ruby-version) }}
     runs-on: ubuntu-latest
     continue-on-error: ${{ matrix.continue-on-error }}
-
+    env:
+      BUNDLE_GEMFILE: ${{ github.workspace }}/${{ matrix.gemfile }}
     steps:
       - uses: actions/checkout@v2