ci: not use our ruby installation script for ruby 2.4

Error:
```
Could not find a valid gem 'bundler' (~> 2.3.0), here is why:
  Unable to download data from https://rubygems.org/ - SSL_connect returned=1 errno=0 state=error: certificate verify failed (https://api.rubygems.org/specs.4.8.gz)
```

See:
ruby/rubygems#1736

Author: nicolastemciuc

.github/workflows/build.yml

@@ -78,15 +78,23 @@ jobs:
     - run: rm Gemfile.lock
 
     - name: Install OpenSSL
+      if: matrix.ruby != '2.4'
       uses: ./.github/actions/install-openssl
       with:
         version: "1.1.1w"
         os: ${{ runner.os }}
 
     - name: Manually set up Ruby
+      if: matrix.ruby != '2.4'
       uses: ./.github/actions/install-ruby
       with:
         version: ${{ matrix.ruby }}
         os: ${{ runner.os }}
 
+    -uses: ruby/setup-ruby@v1
+      if: matrix.ruby == '2.4'
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+
     - run: bundle exec rspec