nixos-rebuild: add explicit option to enable (remote) sudo

Add --use-remote-sudo option. When set, remote commands will be prefixed
with 'sudo'. This allows using sudo remotely _without_ having to use
sudo locally (when using --build-host/--taget-host).

nixos/doc/manual/man-nixos-rebuild.xml

@@ -494,6 +494,20 @@
      </para>
     </listitem>
    </varlistentry>
+
+   <varlistentry>
+    <term>
+     <option>--use-remote-sudo</option>
+    </term>
+    <listitem>
+     <para>
+      When set, nixos-rebuild prefixes remote commands that run on
+      the <option>--build-host</option> and <option>--target-host</option>
+      systems with <command>sudo</command>. Setting this option allows
+      deploying as a non-root user.
+     </para>
+    </listitem>
+   </varlistentry>
   </variablelist>
 
   <para>

nixos/modules/installer/tools/nixos-rebuild.sh

@@ -90,6 +90,11 @@ while [ "$#" -gt 0 ]; do
         targetHost="$1"
         shift 1
         ;;
+      --use-remote-sudo)
+        # note the trailing space
+        maybeSudo="sudo "
+        shift 1
+        ;;
       *)
         echo "$0: unknown option \`$i'"
         exit 1
@@ -97,10 +102,6 @@ while [ "$#" -gt 0 ]; do
     esac
 done
 
-if [ -n "$SUDO_USER" ]; then
-    maybeSudo="sudo "
-fi
-
 if [ -z "$buildHost" -a -n "$targetHost" ]; then
     buildHost="$targetHost"
 fi