poetry: pin virtualenv to 20.30.0 (#439521)

pkgs/by-name/po/poetry/package.nix

···
     {
       poetry = self.callPackage ./unwrapped.nix { };
 
+      # Poetry 2.1.4 officially requires virtualenv >=20.26.6, <20.33.0
+      # otherwise will be incompatible with python312
+      # see: https://github.com/python-poetry/poetry/issues/10490
+      virtualenv = super.virtualenv.overridePythonAttrs (old: rec {
+        version = "20.30.0";
+        src = fetchPypi {
+          inherit (old) pname;
+          inherit version;
+          hash = "sha256-gAhjFivKpUUKbk1yEElzDn8trgdyDgkCsOQEC9b5rag=";
+        };
+      });
+
       # The versions of Poetry and poetry-core need to match exactly,
       # and poetry-core in nixpkgs requires a staging cycle to be updated,
       # so apply an override here.

pkgs/by-name/po/poetry/unwrapped.nix

···
   pythonRelaxDeps = [
     "dulwich"
     "keyring"
-    "virtualenv"
   ];
 
   dependencies = [