scripts/deploy: fix invalid use of "failed" builtin

scripts/deploy

@@ -252,14 +252,18 @@ proc deployOneHost (; host, variant) {
     switch | test {
       info "activating profile... "
       destructive runOnTarget "$netHost" sudo nix-env -p /nix/var/nix/profiles/system --set "$myStorePath"
+
       try {
         destructive runOnTarget "$netHost" sudo "$myStorePath/bin/switch-to-configuration" "$action"
       }
-      var fail = failed
+      var fail = false
+      if failed { setvar fail = true }  # XXX: `failed` special variable is only readable via `if failed`
+
       if (doReboot and (not fail or doRebootForce)) {
         info "rebooting $host"
         destructive runOnTarget "$netHost" sane-reboot "$host" || return 1
       }
+
       if (fail) {
         false
       }