Use standalone conda.exe
We might want to start using conda's standalone exes at https://repo.anaconda.com/pkgs/misc/conda-execs/, as an alternative to downloading and running miniconda.sh and creating a base environment that's otherwise unused.

See https://github.com/conda/conda-prefix-replacement/blob/master/.travis.yml#L9-L14

In theory, we just need to download the right exe for the platform, make it executable, and `conda.exe env create -f ...`

This may be blocked by #28842, since Anaconda doesn't provide 32-bit Linux standalone exes.