fix: Adapt root prefix' precedence for envs_dirs

[holzman]

Adds a new test and fixes #3806. With this PR it now has the desired behavior:

$ mamba create -r /tmp/cliroot --debug -n foo --print-config-only | yq '. | .envs_dirs'
[
  "/tmp/cliroot/envs"
]

$ MAMBA_ROOT_PREFIX=/tmp/envroot mamba create --debug -n foo --print-config-only | yq '. | .envs_dirs'
[
  "/tmp/envroot/envs"
]

$ MAMBA_ROOT_PREFIX=/tmp/envroot mamba create -r /tmp/cliroot --debug -n foo --print-config-only | yq '. | .envs_dirs'
[
  "/tmp/cliroot/envs"
]

$ CONDA_ENVS_DIRS=/tmp/userdirs/envs MAMBA_ROOT_PREFIX=/tmp/envroot mamba create --debug -n foo --print-config-only | yq '. | .envs_dirs'
[
  "/tmp/userdirs/envs",
  "/tmp/envroot/envs"
]

One note - in order for this approach to work, I needed to ensure that the minimum vector size for configuration items was 1 so that --print-config didn't crash. (Alternatively I could change the print-config code to ignore a zero-size vector, but this seemed like a more robust approach).

[jjerphan]

Thank you for your contribution, @holzman!

[holzman]

Ok, I think I've addressed all of the issues, thanks for being so patient!

[jjerphan]

Thank you for your patience.

[jjerphan]

Thank you, @holzman!

I just have a couple of nitpicks but apart from them, the test covers all the cases.

I would wait for another approval from someone in the team before merging this contribution.

[jjerphan]

Suggested change

	for envs_folder in (condarc_envs_dirs, conda_envs_dirs, cli_provided_root, mamba_root_prefix):
	# TODO: Remove those calls to `os.makedirs` once
	# https://github.com/mamba-org/mamba/issues/3790 is fixed
	for envs_folder in (condarc_envs_dirs, conda_envs_dirs, cli_provided_root, mamba_root_prefix):

[holzman]

Ok, I've added one more commit to address the nits. (Now I'm off to write a better test for #3796).