runCommand: don't set meta.position if meta is given

Fixes #242093 without breaking #237589

If `meta` is given we probably already have position information, so no
need to get it from the arguments.

pkgs/build-support/trivial-builders/default.nix

@@ -87,6 +87,8 @@ rec {
       inherit buildCommand name;
       passAsFile = [ "buildCommand" ]
         ++ (derivationArgs.passAsFile or []);
+    }
+    // lib.optionalAttrs (! derivationArgs?meta) {
       pos = let args = builtins.attrNames derivationArgs; in
         if builtins.length args > 0
         then builtins.unsafeGetAttrPos (builtins.head args) derivationArgs