flake: explicitly configure commit-lock-file-summary

Sets `nixConfig.commit-lock-file-summary` to "flake: Update", matching
the default used by `ci/update`. This ensures consistency when running
`nix flake update --commit-lock-file` or `nix flake lock --commit-lock-file`
manually.

Note: manual updates are discouraged. The `nix-build ci -A update`
script should be used to update flakes. This ensures both root and dev
lockfiles — as well as `version-info.toml` — are kept in sync.

Author: MattSturgeon

flake.nix

@@ -18,6 +18,7 @@
   };
 
   nixConfig = {
+    commit-lock-file-summary = "flake: Update";
     extra-substituters = [ "https://nix-community.cachix.org" ];
     extra-trusted-public-keys = [
       "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="