Try using a workaround to add allow_failure to GitHub Actions.

I think this'll work?

.github/workflows/ruby.yml

@@ -35,12 +35,14 @@ jobs:
       fail-fast: false # don't fail all matrix builds if one fails
       matrix:
         ruby:
-          - '2.5' 
-          - '2.6' 
+          - '2.5'
+          - '2.6'
           - '2.7'
           - '3.0'
-          - 'head'
           - truffleruby-head
+        include:
+          - ruby: 'head'
+            allow_failure: true
     continue-on-error: ${{ endsWith(matrix.ruby, 'head') }}
     steps:
     - uses: actions/checkout@v2
@@ -67,3 +69,9 @@ jobs:
       run: |
         export GIT_BRANCH="${GITHUB_REF/refs\/heads\//}"
         ./cc-test-reporter after-build -r $CC_TEST_REPORTER_ID
+    # Hack to get around, it runs `true` which will return a successful
+    # exit code no matter what. Remove this once ruby-head is passing again
+    # later.
+    - name: ignore failure if the job is allowed to fail
+      if: ${{ matrix.allow_failure }}
+      run: true