Explicitly choose main as the branch to fetch from core-sans-a-fonts repo

The build failed to find the default branch for it, on repo checkout

Author: oliverbarnes

.github/workflows/ci.yml

@@ -19,6 +19,7 @@ jobs:
         with:
           fetch-depth: 0
           repository: mainmatter/core-sans-a-fonts
+          ref: main
           ssh-key: ${{ secrets.SSH_PRIVATE_KEY }}
           path: core-sans-a-fonts
       - name: Install Fonts