Manual triggers of mirror workflows do not work

[jobara]

Prerequisites

• I've searched for existing issues

Describe the bug

After failing to automatically run the mirror-staging workflow ( see: #2361 ) I attempted to manually trigger the mirror staging workflow. However, that failed related to an error with pulling. Initially I thought that was an issue with the configuration of the pull but on closer inspection it's likely related to the workflow not specifying the base branch. The checkout action will likely sort this out based on the push event, but when called manually it is unlikely to know which reference triggered it. It's likely related to actions/checkout#1418

To Reproduce

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4. See error

Expected behavior

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Additional context

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