Add a keep_case flag which ensures that env variables are not uppercased

[ForgottenProgramme]

Description

resolves #13713

At present conda converts all environment variables into uppercase.
This PR adds a keep_case context variable, which allows users to store their environment variables as is.

When the --keep-case flag is used, environment variables are saved in "preserved case". When the flag is not used variables are saved in upper case (the original conda behavior).

Tasks

• Add a new keep_case variable to context
• store environment variables in preserved case when keep_case is True
• write tests
• write more tests
• Parametrize the tests!

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[codspeed-hq]

CodSpeed Performance Report

Merging #13904 will not alter performance

_{Comparing ForgottenProgramme:env-vars (a9d3dab) with main (eb45954)}

Summary

✅ 21 untouched benchmarks

[ForgottenProgramme]

pre-commit.ci autofix

[jezdez]

this would be even better with a more descriptive name for the config option, keep case of what?

[jezdez]

Closed in favor of #13943.