stdenv: set enableParallelBuilding explicitly if enableParallelBuildingByDefault is set

Without the change we don't propagate `enableParallelBuilding = true`
and leave most builds sequential.

Noticed on `mythtv` package which did not specify parallelism and
`config.enableParallelBuildingByDefault = true` had no effect.

pkgs/stdenv/generic/make-derivation.nix

@@ -418,6 +418,7 @@ else let
       outputHashAlgo = attrs.outputHashAlgo or "sha256";
       outputHashMode = attrs.outputHashMode or "recursive";
     } // lib.optionalAttrs (enableParallelBuilding) {
+      inherit enableParallelBuilding;
       enableParallelChecking = attrs.enableParallelChecking or true;
     } // lib.optionalAttrs (hardeningDisable != [] || hardeningEnable != [] || stdenv.hostPlatform.isMusl) {
       NIX_HARDENING_ENABLE = enabledHardeningOptions;