Add search algorithm for environments.txt file (remove hardcoded path)

[jrom99]

Description

As seem on #13453, the environments.txt file is a configuration file that is currently hardcoded to ~/.conda/environments.txt and is one (among many) files responsible for the creation of ~/.conda. This commit tries to reduce the dependency on that directory.

The current behavior of get_user_environments_txt_file is ambiguous in that it may return a file to write (default behavior) or multiple files to read (during the execution of list_all_known_prefixes). This commit adds a search algorithm for it so that it is not written to ~/.conda by default (but it's still able to find it there), and adds a docstring specifying the "file to write" behavior.

This commit expands list_all_known_prefixes search algorithm to include all config paths specified in https://docs.conda.io/projects/conda/en/latest/user-guide/configuration/use-condarc.html#searching-for-condarc.

One should be aware that another user on the same computer may choose to set their $XDG_CONFIG_HOME to a value that is different from $HOME/.config. Since this was not dealt with before (and I'm not sure what should be the expected behavior when one lists environments with admin privileges), I choose not to implement this.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes? No.
• Add / update necessary tests? No.
• Add / update outdated documentation? No.

[conda-bot]

We require contributors to sign our Contributor License Agreement and we don't have one on file for @jrom99.

In order for us to review and merge your code, please e-sign the Contributor License Agreement PDF. We then need to manually verify your signature, merge the PR (conda/infrastructure#896), and ping the bot to refresh the PR.

[bigodel]

adding a +1 to this feature