add a knob to inject git branch into `CacheKey`

[pbjhelmert]

In my Rocky images, I'd like to set a DNF variable based on the Git branch name so that I can use differing DNF repositories based on the current Git branch. Unfortunately, there's no way to hint this to mkosi's caching, and different branches would incorrectly appear to be using the same repository.

Can we add a way to inject the Git branch into CacheKey to accommodate this scenario?