Inline the build

Let's see if this avoids the issues we're seeing in CI.

Author: PeterJCLaw

.github/workflows/validate.yml

@@ -15,6 +15,8 @@ jobs:
       BUNDLE_PATH: gems
     steps:
     - uses: actions/checkout@v2
+      with:
+        submodules: recursive
 
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
@@ -24,4 +26,4 @@ jobs:
         cache-version: 1
 
     - name: Build
-      run: bundle exec rake build
+      run: bundle exec jekyll build --config _config.yml