fix deploy-rs: add StrictHostKeyChecking to sshOpts

deploy-rs ignores NIX_SSHOPTS and only uses its own sshOpts.
The host key verification prompt was blocking the non-interactive
CI pipeline. Adding accept-new to sshOpts directly.

flake.nix

@@ -127,6 +127,8 @@
             sshOpts = [
               "-p"
               "2299"
+              "-o"
+              "StrictHostKeyChecking=accept-new"
             ];
             profiles.system = {
               user = "root";