Fix custom environment variables logic

[rdewit]

In the current version ogr2ogr, custom environment variables get ignored due to a bug in the code.

This PR aims to fix this and adds relevant tests as well as updated documentation.

[wavded]

Thanks for looking into this! Please fix the linting errors and should be good to go.

[rdewit]

@wavded, thanks for the review. I fixed the styling issues by running npm run fmt and subsequently npm run fmt-check.

Note: on my Windows machine, running prettier actually changes all the files and a git diff shows something like the following for every file:

warning: LF will be replaced by CRLF in changelog.md.
The file will have its original line endings in your working directory

Adding the option "endOfLine": "auto" in .prettierrc will fix this but I'm not sure of unintended consequences.

Either way, fingers crossed that the checks will pass now.

[wavded]

Checks passed! Thx @rdewit for the contrib! Merged