stdenv.mkDerivation: check meta.mainProgram from original meta

We define a meta variable in the same let block as the result of
the `commonMeta` call. Avoiding evaluating the result of that call
until we need to is preferable.

pkgs/stdenv/generic/make-derivation.nix

@@ -866,7 +866,8 @@ let
       );
 
     let
-      env' = if meta ? mainProgram then env // { NIX_MAIN_PROGRAM = meta.mainProgram; } else env;
+      env' =
+        if attrs ? meta.mainProgram then env // { NIX_MAIN_PROGRAM = attrs.meta.mainProgram; } else env;
 
       derivationArg = makeDerivationArgument (
         removeAttrs attrs [