don't install private fonts on forks

.github/workflows/ci.yml

@@ -15,13 +15,16 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
-      - uses: actions/checkout@v4
+      - name: Get Core Sans
+        uses: actions/checkout@v4
+        if: github.repository_owner == 'mainmatter'
         with:
           fetch-depth: 0
           repository: mainmatter/core-sans-a-fonts
           ssh-key: ${{ secrets.SSH_PRIVATE_KEY }}
           path: core-sans-a-fonts
       - name: Install Fonts
+        if: github.repository_owner == 'mainmatter'
         run: |
           sudo cp -r core-sans-a-fonts/* /usr/local/share/fonts/
           sudo fc-cache -f -v