Add bundle exec to build command

Author: JonRowe

.github/workflows/middleman_deploy.yml

@@ -55,7 +55,7 @@ jobs:
       - name: Build with Middleman
         run: |
           mkdir -p __source_build
-          BUILD_OUTPUT="__source_build" middleman build
+          BUILD_OUTPUT="__source_build" bundle exec middleman build
       - name: Set CNAME
         if: ${{ github.repository == 'rspec/rspec.github.io' }}
         run: echo "rspec.info" > CNAME