Add a workaround for plutus-tx-plugin

Looks like `plutus-tx-plugin` depends on `ghc` package and it seems that
new dependencies brought in with `cardano-crypto-class-2.2` create a
conflict due to `stm` version used with `ghc-9.6` being incompatible.

flake.nix

@@ -80,6 +80,8 @@
           '';
 
           shell = {
+            # Due to plutus-tx-plugin being a bit special, we need to augment the default package selection.
+            packages = ps: builtins.attrValues (nixpkgs.haskell-nix.haskellLib.selectLocalPackages ps) ++ [ps.plutus-tx-plugin];
 
             # force LANG to be UTF-8, otherwise GHC might choke on UTF encoded data.
             shellHook = ''