explicitly install nokogiri 1.15.5

It's the last version of nokogiri that works with ruby 2.7.

Author: travisdowns

.github/workflows/ci.yml

@@ -124,7 +124,9 @@ jobs:
 
     - name: Install htmlproofer
       if: ${{ ! matrix.script.no-html-proofer }}
-      run: sudo gem install html-proofer -v 3.19.4 --no-document
+      run: |
+        sudo gem install nokogiri -v 1.15.5 --no-document
+        sudo gem install html-proofer -v 3.19.4 --no-document
 
     - name: Run ${{ matrix.script.path }}
       run: main/_scripts/checks/${{ matrix.script.path }}