don't install private fonts on forks

Author: marcoow

.github/workflows/ci.yml

@@ -15,7 +15,9 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
-      - uses: actions/checkout@v4
+      - name: Get Core Sans
+        uses: actions/checkout@v4
+        if: github.event.repository.fork == false
         with:
           fetch-depth: 0
           repository: mainmatter/core-sans-a-fonts