move kexec check into kexec codepath

without this, the check gets run and fails even if no kexec will be done
e.g. when an installer is booted

Author: ajs124

src/nixos-anywhere.sh

@@ -301,11 +301,11 @@ if [[ ${is_os-n} != "Linux" ]]; then
   abort "This script requires Linux as the operating system, but got $is_os"
 fi
 
-if [[ ${is_arch-n} != "x86_64" ]] && [[ $kexec_url == "$default_kexec_url" ]]; then
-  abort "The default kexec image only support x86_64 cpus. Checkout https://github.com/numtide/nixos-anywhere/#using-your-own-kexec-image for more information."
-fi
-
 if [[ ${is_kexec-n} == "n" ]] && [[ ${is_installer-n} == "n" ]]; then
+  if [[ ${is_arch-n} != "x86_64" ]] && [[ $kexec_url == "$default_kexec_url" ]]; then
+    abort "The default kexec image only support x86_64 cpus. Checkout https://github.com/numtide/nixos-anywhere/#using-your-own-kexec-image for more information."
+  fi
+
   step Switching system into kexec
   ssh_ <<SSH
 set -efu ${enable_debug}