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.

cabal.project

@@ -24,9 +24,9 @@ source-repository-package
   -- !WARNING!:
   -- MAKE SURE THIS POINTS TO A COMMIT IN `MAlonzo-code` BEFORE MERGE!
   subdir: generated
+  --sha256: sha256-by921yC1MaZI58kyrtEGGKqmt9jMnVsPLfQtP4raJPw=
   tag: be51adff014214c15fed718d396bb8a6d955f9e1
 
---sha256: sha256-by921yC1MaZI58kyrtEGGKqmt9jMnVsPLfQtP4raJPw=
 -- NOTE: If you would like to update the above, look for the `MAlonzo-code`
 -- branch in the `formal-ledger-specifications` repo and copy the SHA of
 -- the commit you need. The `MAlonzo-code` branch functions like an alternative

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 = ''