fix: nina back uses switchGeneration on flake systems, not --rollback

@@ -140,7 +140,13 @@140140     output.backStart(ctx.machine.name, current, prev);141141     if (ctx.cfg.confirm and !output.confirm("")) return;142142 143-    const code = try exec.rollback(ctx.io, &ctx.machine, ctx.gpa);143+    // Flake systems don't have <nixos-config> in the Nix search path, so144+    // `nixos-rebuild --rollback` fails. Switch by generation number instead145+    // (same path as `nina go N`), which works for both channel and flake.146+    const code = if (flakeSystem(ctx))147+        try exec.switchGeneration(ctx.io, &ctx.machine, ctx.gpa, prev)148+    else149+        try exec.rollback(ctx.io, &ctx.machine, ctx.gpa);144150     if (code != 0) return error.BuildFailed;145151     output.backDone(prev);146152     runPostHook(ctx, "post-back");