This commit allows the `forgejo-cli actions register` command to change
an existing runner's secret, as discussed in #4610.
It refactors `RegisterRunner` to extract the code that hashes the token,
moving this code to a method called `UpdateSecret` on `ActionRunner`.
A test for the method has been added.
The `RegisterRunner` function is updated so that:
- it relies on `ActionRunner.UpdateSecret` when creating new runners,
- it checks whether an existing runner's secret still matches the one
passed on the command line,
- it updates the runner's secret if it wasn't created and it no longer
matches.
A test has been added for the new behaviour.
320ab7ed7f