Fixed libmamba crash when using pip

[Klaim]

Also added an overload of env::which taking a list of search paths for the executable to find.

[chaubold]

I'm sorry that I introduced that crash on Windows with my fix in #1815, but @Klaim your fix actually re-introduces the problem that installing packages with pip in environments with spaces in their path does not work 😞 #1805.

Now, using micromamba=0.25.1 the Python executable used for the pip installation process is not found if the path to Python contains a space. At least on Mac. If I use micromamba=0.25.0 then it works.

[Klaim]

@chaubold Ouch, sorry about that, I wasn't aware of that other issue. I'll make a fix so that both issues work. And add a test for it, that would have helped me realise my mistake XD

[chaubold]

Yeah, I should have added a test in the first place... Sorry about that.
If you set up a fix that would be awesome, please ping me in the PR, I'm happy to try it out and help reviewing.

[Klaim]

@chaubold #1838 should fix it, also added a test