build(flake): add convco to dev

Adds the tool convco to the flake, allowing eza hackers to run
`convco commit`, giving a guided flow for creating conventional commits.

Refs: 149
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 vhs ];
+          nativeBuildInputs = with pkgs; [ toolchain just pandoc vhs convco ];
         };
 
         # for `nix flake check`