Fix detect_architecture print_warn captured by command substitution

Redirect warning to stderr so it doesn't pollute the arch variable
when called as arch=$(detect_architecture) on unsupported platforms.

Author: SamNet-dev

dnstm-setup.sh

@@ -1506,7 +1506,7 @@ detect_architecture() {
             echo "armv7"
             ;;
         *)
-            print_warn "Unsupported architecture: $machine_arch (defaulting to amd64)"
+            print_warn "Unsupported architecture: $machine_arch (defaulting to amd64)" >&2
             echo "amd64"
             ;;
     esac