build: add charm to nix develop

Signed-off-by: Christina Sørensen <christina@cafkafk.com>

flake.nix

@@ -71,7 +71,7 @@
 
         # For `nix develop`:
         devShells.default = pkgs.mkShell {
-          nativeBuildInputs = with pkgs; [toolchain just pandoc];
+          nativeBuildInputs = with pkgs; [ toolchain just pandoc vhs ];
         };
 
         # for `nix flake check`