Fix #5925 Add file-watch-hook for post-processing

[mpilgrem]

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Please also shortly describe how you tested your change. Bonus points for added tests!

This is draft while testing on different operating systems.

[mpilgrem]

--file-watch is now misbehaving. I think this because I lifted the withConfig up out of the inner (because fileWatchConf itself needed access to the file-watch-hook configuration). I'll reverse that.