MAMBA_ROOT_PREFIX has precedence over -r option

[holzman]

This issue was introduced by #3692:

In 2.0.5, -r took precedence over MAMBA_ROOT_PREFIX:

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

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

but in 2.0.6:

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

Note that this only matters when /tmp/envroot/envs already exists.

Originally posted by @holzman in #3796 (comment)

[holzman]

I added a parameter in a test case for this:

holzman@e7d1398