forgejo-ci: use webfactory/ssh-agent for ssh logics

.forgejo/workflows/push.yaml

@@ -7,9 +7,10 @@ jobs:
     steps:
       - name: Environment setup
         run: |
-          apk add git nodejs
-          mkdir -p ~/.ssh
-          echo ${{ secrets.PAGES_PRIVKEY }} >  ~.ssh/id_ed25519
+          apk add git nodejs ssh-agent
+      - uses: https://github.com/webfactory/ssh-agent@v0.9.0
+        with:
+          ssh-private-key ${{ secrets.PAGES_PRIVKEY }}
       - name: Repo pull
         uses: actions/checkout@v4
         with: