Trust Tangled host key during mirror

.github/workflows/mirror-to-tangled.yml

···
         with:
           ssh-private-key: ${{ secrets.TANGLED_DEPLOY_KEY }}
 
+      - name: Add Tangled host key
+        run: |
+          mkdir -p ~/.ssh
+          ssh-keyscan -H tangled.sh >> ~/.ssh/known_hosts
+
       - name: Push to Tangled
         env:
           TARGET_REMOTE: [e-mail]:galiglobal.com/frontpage_firefox_plugin