rtfcv

changing Path environment variable after shell starts have no effect on windows

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### Describe the bug Changing path variable using let-env command after shell starts has no effect what-so-ever. Another nu.exe started from that shell will take the new env into account....

The title says it all. The problem can be easily reproduced in the rather sloppy code below. I think the problem is simple, as I also wrote in pull request...

moved update of last_generation inside if(!user_request_stop) block for solve_next_generation function This should prevent last_generation instance from being overwritten by incomplete new_generation