CVE-2023-31128: NextCloud Cookbook's pull-checks.yml workflow is vulnerable to OS Command Injection

NextCloud Cookbook is a recipe library app. Prior to commit a46d9855 on the master branch and commit 489bb744 on the main-0.9.x branch, the pull-checks.yml workflow is vulnerable to command injection attacks because of using an untrusted github.headref field. The github.headref value is an attacker-controlled value. Assigning the value to zzz";echo${IFS}"hello";# can lead to command injection. Since the permission is not restricted, the attacker has a write-access to the repository. This issue is fixed in commit a46d9855 on the master branch and commit 489bb744 on the main-0.9.x branch. There is no risk for the user of the app within the NextCloud server. This only affects the main repository and possible forks of it. Those who have forked the NextCloud Cookbook repository should make sure their forks are on the latest version to prevent code injection attacks and similar.