doc: update command-ref/env-common.md

[phip1611]

Add more details to make clearer how it works.

[fricklerhandwerk]

There was a recent change that improved the wording and examples, but moved the information in this section to documentation pages for each nix command, e.g. nix build. It left a dangling verbal reference (stop-gap fix proposed in #7514).

I think we should still fix that in a better way, and instead move the description of how search paths work back to NIX_PATH documentation, such that it can be linked to from both old-style and new-style commands without duplicating the source. Then we can continue working on examples.

It wouldn't be as convenient for readers because they'd have to follow an anchor link or search the man page to read the environment variables section, but easier than somehow rendering the new in-code documentation into the existing markdown pages.

[phip1611]

I moved an additional example from #7629 to this PR, as it fits better into the new location. What do you think?

PS: Oh, as you mentioned, I just saw that an example was there before but is removed now:

https://github.com/NixOS/nix/pull/7421/files#diff-e0fbdd0cb194b7d08dad9f619bff3659613a1de50b30a357501e54bac94edda1

Feel free to close my PR if you think that the solution is more complex and out of scope of this small PR.

[fricklerhandwerk]

Yes, I'll close this one in favor of a more elaborate solution as described above. Feel free to propose one if you find time, this will surely make it happen faster than waiting for me or anyone else. Thanks a lot for your time, and for giving things a thought.