chore: add deploy_key for gk repo

Author: staaldraad

.github/workflows/nix-build.yml

@@ -52,6 +52,12 @@ jobs:
           sudo -E python -c "import os; file = open('/etc/nix/nix-secret-key', 'w'); file.write(os.environ['NIX_SIGN_SECRET_KEY']); file.close()"
         env:
           NIX_SIGN_SECRET_KEY: ${{ secrets.NIX_SIGN_SECRET_KEY }}
+      - name: Setup SSH for deploy key
+        run: |
+          mkdir -p ~/.ssh
+          echo "${{ secrets.GK_DEPLOY_KEY }}" > ~/.ssh/id_ed25519
+          chmod 600 ~/.ssh/id_ed25519
+          ssh-keyscan github.com >> ~/.ssh/known_hosts
       - name: Setup cache script
         if: ${{ github.secret_source == 'Actions' }}
         run: |