haskell.packages.ghc912.extensions: fix the eval

Without the change `haskell.packages.ghc912.extensions` eval fails as:

    $ nix build --no-link -f. haskell.packages.ghc912.extensions
    error:
       … in the left operand of the update (//) operator
         at pkgs/development/haskell-modules/lib/compose.nix:41:6:
           40|     f: drv:
           41|     (drv.override (
             |      ^
           42|       args:

       error: attribute 'extensions_0_1_0_3' missing
       at pkgs/development/haskell-modules/configuration-ghc-9.12.x.nix:75:29:
           74|
           75|   extensions = doDistribute self.extensions_0_1_0_3;
             |                             ^
           76|   ghc-exactprint = doDistribute self.ghc-exactprint_1_12_0_0;
       Did you mean one of extensions_0_1_0_1 or extensions_0_1_1_0?