nix: show stderr output for postgrest-style

Hiding this will also hide errors when they happen...

In my case, my nix-shell's TMPDIR was too long, so black was failing
silently because it couldn't create a unix socket for its forkserver.

nix/tools/style.nix

@@ -25,14 +25,14 @@ let
       ''
         # Format Nix files
         ${statix}/bin/statix fix
-        ${nixpkgs-fmt}/bin/nixpkgs-fmt . > /dev/null 2> /dev/null
+        ${nixpkgs-fmt}/bin/nixpkgs-fmt .
 
         # Format Haskell files
         ${fd}/bin/fd '\.l?hs$' \
           | xargs ${stylish-haskell}/bin/stylish-haskell -i
 
         # Format Python files
-        ${black}/bin/black . 2> /dev/null
+        ${black}/bin/black .
       '';
 
   # Script to check whether any uncommitted changes result from postgrest-style