ci: install graphviz in Jepsen CI for Elle graph analysis

Elle requires graphviz (dot) to render consistency graphs.
Without it, Jepsen reports {:valid? :unknown} instead of a
definitive pass/fail result.

Author: bootjp

.github/workflows/jepsen-test.yml

@@ -22,8 +22,8 @@ jobs:
       - uses: actions/setup-go@v6
         with:
           go-version-file: 'go.mod'
-      - name: Install netcat
-        run: sudo apt-get update && sudo apt-get install -y netcat-openbsd
+      - name: Install netcat and graphviz
+        run: sudo apt-get update && sudo apt-get install -y netcat-openbsd graphviz
       - name: Install Leiningen
         run: |
           curl -L https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein > ~/lein