github: set up .ruby-version + cache dependencies

- set up Ruby according to .ruby-version
- cache Ruby deps

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

.github/workflows/build.yml

@@ -27,9 +27,11 @@ jobs:
     - uses: actions/setup-python@v5
       with:
         python-version: 3.9
+    - uses: ruby/setup-ruby@v1
+      with:
+        bundler-cache: true
     - run: pip3 install -U camkes-deps
     - run: sudo apt-get install doxygen
-    - run: sudo gem install bundler -v 2.4.22
     - run: make build JEKYLL_ENV=production
     - run: tar -cvf site.tar _site/
     - uses: actions/upload-artifact@v4