fix: use custom devShell to avoid rebuilds on source changes

It's rebuilding because we're using IFD to build the Cargo.nix, which is an input to the Centipede package.
Maybe this can be fixed by only using ./Cargo.lock as an input to the generatedCargoNix call?

flake.nix

@@ -47,5 +47,13 @@
           centipede = cargoNix.workspaceMembers.centipede.build;
           default = centipede;
         };
+
+        devShell = pkgs.mkShell {
+          name = "centipede";
+
+          packages = [
+            rust
+          ];
+        };
       });
 }