PR body uses hardcoded branch name

[srid]

The branch name update_flake_lock_action is hardcoded here:

update-flake-lock/action.yml

Lines 47 to 51 in 222f041

	git branch -D update_flake_lock_action
	git fetch origin
	git checkout update_flake_lock_action
	git commit --amend --no-edit
	git push origin update_flake_lock_action --force

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:

srid/haskell-flake#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.

srid/nixos-config@db392e4