Fix mamba not writable cache error

[adriendelsalle]

Description

Fix mamba runtime_error when root_prefix cache in not writable
Use the first writable cache dir in Channel constructor to re-use Context.pkgs_dirs filled using conda api

The bug has been introduced in #1033, by using libmamba Channel instead of conda's one.

Closes #1098

[adriendelsalle]

@wolfv any idea why the tests you introduced recently are failing on Windows?

Will try to debug that if you don't have time to have a look.
I just triggered again the 0.15.2 release workflow to be sure it still passes.

[adriendelsalle]

The exact same tests are failing on 0.15.2 workflow re-ran: https://github.com/mamba-org/mamba/runs/3233778749?check_suite_focus=true