use_condaenv() fixes

[t-kalinowski]

This patch allows reticulate to bind to a condaenv where the original conda binary that was used to create the condaenv is no longer available. In that case, reticulate will fall back to the same search path as conda_binary("auto").

Note, we still require that a conda binary is available on the system to use a condaenv. Reticulate uses this to call the conda activate command.
The relevant "Finding Conda" section in ?conda_binary is reproduced here:

When conda = "auto", reticulate will attempt to automatically find a conda installation. The following locations are searched, in order:

1. The location specified by the reticulate.conda_binary \R option,
2. The location specified by the RETICULATE_CONDA environment variable,
3. The [miniconda_path()] location (if it exists),
4. The program PATH,
5. A set of pre-defined locations where conda is typically installed.

To force reticulate to use a particular conda binary, we recommend
setting:

options(reticulate.conda_binary = "/path/to/conda")

Closes #1542, #1460
Related #1176

[mtekman]

Would it be possible to patch this more such that setting:

options(reticulate.conda.binary = FALSE)

would force a system python?