run ssh commands thru the shell (which has openssh) (#461)

Reviewed-on: Fediversity/Fediversity#461
Co-authored-by: Kiara Grouwstra <kiara@procolix.eu>
Co-committed-by: Kiara Grouwstra <kiara@procolix.eu>

.forgejo/workflows/cd.yaml

@@ -19,8 +19,8 @@ jobs:
           mkdir -p ~/.ssh
           echo "${{ secrets.CD_SSH_KEY }}" > ~/.ssh/id_ed25519
           chmod 600 ~/.ssh/id_ed25519
-          eval "$(ssh-agent -s)"
-          ssh-add ~/.ssh/id_ed25519
+          nix-shell --run 'eval "$(ssh-agent -s)"'
+          nix-shell --run 'ssh-add ~/.ssh/id_ed25519'
 
       - name: Deploy
         run: nix-shell --run 'nixops4 apply default'