feat: add mirroring workflow

.github/workflows/mirror.yml

@@ -0,0 +1,17 @@
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - env:
+          GIT_SSH_COMMAND: "ssh -i id_ed25519"
+        run: |
+          printf '${{ secrets.SRHT_MIRROR_PRIVATE_KEY }}' > id_ed25519
+          ssh-keyscan -t rsa github.com >> ~/.ssh/known_hosts
+          git remote add srht [email protected]:~beleap/nixos-configuration
+          git push --mirror srht