devshell: remove dependency on clan-cli

This stops the devshell from being re-built constantly, when the source code changes.

Based on a new feature in treefmt-nix: https://github.com/numtide/treefmt-nix/pull/208

devShell.nix

@@ -38,6 +38,7 @@
         ];
         shellHook = ''
           echo -e "${ansiEscapes.green}switch to another dev-shell using: select-shell${ansiEscapes.reset}"
+          export PROJECT_ROOT=$(git rev-parse --show-toplevel)
         '';
       };
     };