[core][runtime_env] Allow full path in conda runtime env.

[rynewang]

#34956 fixed a conda hanging issue by checking existence of the conda env name. However it only checks by conda env name, not path - if the user specifies an absolute path, it no longer works. This is an regression because it should support full paths without a problem.

This PR reads conda CLI for the full paths and names. A special treatment is done for base which is just the conda base prefix; and others are base conda_prefix/envs/name.

Adds tests for both base and full path of it; and also name and full paths for non base envs.

Fixes #44373 .