Use webfactory/ssh-agent instead of file-based key

Author: hinerm

.github/setup.sh

@@ -11,10 +11,9 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
+      - uses: webfactory/ssh-agent@v0.5.3
+         with:
+             ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }
 
-      - name: Set up CI environment
-        run: .github/setup.sh
       - name: Execute the build
         run: .github/build.sh
-        env:
-          SSH_PRIVATE_KEY: ${{ secrets.SSH_PRIVATE_KEY }}