reduce commit message size for flake updates

Signed-off-by: ahuston-0 <aliceghuston@gmail.com>

.github/workflows/flake-update.yml

@@ -112,7 +112,11 @@ jobs:
                 commit-message: |
                     automated: Update `flake.lock`
 
-                    ${{ steps.pr_body.outputs.content }}
+                    Auto-generated by [update.yml][1] with the help of
+                    [create-pull-request][2].
+
+                    [1]: https://nayeonie.com/ahuston-0/nix-dotfiles/src/branch/main/.github/workflows/flake-update.yml
+                    [2]: https://forgejo.stefka.eu/jiriks74/create-pull-request
                 branch: update-flake-lock
                 delete-branch: true
                 pr-labels: | # Labels to be set on the PR