Speed up mk_all check in CI by fetching Mathlib cache before running it

Co-Authored-By: Claude <noreply@anthropic.com>

Author: SnO2WMaN

.github/workflows/ci.yml

@@ -104,6 +104,8 @@ jobs:
         run: |
           curl https://raw.githubusercontent.com/leanprover/elan/master/elan-init.sh -sSf | sh -s -- -y
           echo "$HOME/.elan/bin" >> "$GITHUB_PATH"
+      - name: Get Mathlib cache
+        run: lake exe cache get
       - name: Check mk_all executed
         run: |
           just mk-all