stack-to-nix: Prune cache file

[rvl]

The .stack-to-nix.cache file only ever grows. Old entries should be cleaned up.

[chreekat]

I suppose it would be a bit harsh to prune it each time during development -- I'm thinking of typos and rollbacks. So perhaps there should be a --prune argument to make it an explicit choice?

[stale]

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