Simplify environment variable examples

I’ve found myself visiting the documentation for `std::env::vars`
every few months, and every time I do, it is because I want to quickly
get a snippet to print out all environment variables :-)

So I think it could be nice to simplify the examples a little to make
them self-contained. It is of course a style question if one should
import a module a not, but I personally don’t import modules used just
once in a code snippet.

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