Use system Ruby version

Author: manicmaniac

.github/workflows/test.yml

@@ -58,7 +58,7 @@ jobs:
       run: sed -i '' -E '/apollographql\/apollo-ios/s/"[0-9.]+"/"${{ matrix.apollo }}"/' Cartfile.resolved
     - uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.6
+        ruby-version: system
         bundler-cache: true
       env:
         BUNDLE_WITHOUT: development:documentation
@@ -99,7 +99,7 @@ jobs:
     - uses: actions/checkout@v2
     - uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.6
+        ruby-version: system
         bundler-cache: true
       env:
         BUNDLE_WITHOUT: documentation:test