nu: Fix "m daemon --boringtun"

tree/.local/nu/mullvad.nu

@@ -84,6 +84,10 @@ def "m daemon" [
         ]
     }
 
+    if $boringtun {
+        $cargo_flags ++= [--features boringtun]
+    }
+
     if (m daemon is-running --system) {
         print "daemon is loaded, stopping..."