buildCrystalPackage: fix env overrides

Passing variables through `env` was broken for 2 reasons:

1. due to missing parens, no merging of `env` would take place, and
2. it was impossible to override defaults due to the order

pkgs/development/compilers/crystal/build-package.nix

@@ -108,11 +108,12 @@ stdenv.mkDerivation (
         ++ [ "runHook postConfigure" ]
       ));
 
-    env = args.env or { } // {
+    env = {
       CRFLAGS = lib.concatStringsSep " " defaultOptions;
 
       PREFIX = placeholder "out";
-    };
+    }
+    // (args.env or { });
 
     inherit enableParallelBuilding;
     strictDeps = true;
@@ -205,8 +206,9 @@ stdenv.mkDerivation (
         done
       '';
 
-    meta = args.meta or { } // {
-      platforms = args.meta.platforms or crystal.meta.platforms;
-    };
+    meta = {
+      platforms = crystal.meta.platforms;
+    }
+    // (args.meta or { });
   }
 )