Cache remote repositories

[mxmehl]

Remote repositories should be cached somehow, so that they don't need to be checked out again with each invocation of the checker. We could check them out to a well-defined space (e.g. ~/.local/state/contribution-checker/repos) and then just run a git pull if the checkout already exists.

https://github.com/platformdirs/platformdirs could be used to get the corresponding cache directory for all systems.