Windows: Fixed environment variables not read as unicode.

[Klaim]

The code acquiring environment variables on Windows was doing so using the non-unicode API which lead to some environment variables containing unicode path (or other unicode values) leading to random crashes.
This version uses a simpler API call which is unicode-enabled and should fix #2153

[Klaim]

For info I'm trying to rewrite the solution with the safe functions and to handle the setenv because otherwse tests cannot pass.

[AntoinePrv]

Thanks 🎉

Can we take this opportunity to add some libmamba tests? Since we have env::get and env::set we can at least test them together.

Testing env::get alone would require setting an env var from outside the test exe. Not perfect, but maybe necessary?

[AntoinePrv]

Nit: One can also do (with the right header)

Suggested change

	LOG_ERROR << fmt::format(
	"Failed to acquire environment variable '{}' : errcode = {}",
	key,
	error_code
	);
	fmt::format_to(LOG_ERROR,
	"Failed to acquire environment variable '{}' : errcode = {}",
	key,
	error_code
	);

[Klaim]

BTW I know this is possible but I dont know if that changes something performance-wise for fmt? Do you know?

[Klaim]

Actually I tried that syntax and it doenst seem to compile, probably lacks some compatibility layer to add. I'll keep it as is for now.

[Klaim]

@AntoinePrv I can add more tests although note that I had to change set because current C++ tests (test_libmamba) have tests that use the environment and check that they are getting what is expected (they were failing with my first commit) - but they are not testing these specific functions.

[Klaim]

I will also add a basic test for the unicode functions.

[Klaim]

@AntoinePrv I added basic tests using mamba::env::get to get environment variables that are always available on windows, probably will also fail if the user running the test is using a unicode path (we migth want to do that on the CI too...).
If you want to add another test for linux be my guest, I cant check right now such code as I'm stuck on windows until next week.

[AntoinePrv]

Thanks!