PR body uses hardcoded branch name

[srid]

The branch name update_flake_lock_action is hardcoded here:

https://github.com/DeterminateSystems/update-flake-lock/blob/222f041780a88f7f379bd7a3c3327585e435db5b/action.yml#L47-L51

But this becomes incorrect, when the user overrides it in the workflow YAML as I do here. This results in creating PRs with wrong instructions:

https://github.com/srid/haskell-flake/pull/321

The PR body refers to branch update_flake_lock_action contradicting the actual branch update_flake_lock_example.

[srid]

Aside: I wrote this little Nu script to automate the steps, taking the PR URL directly.

https://github.com/srid/nixos-config/commit/db392e491722722fdb687f6da698e42492423fb2