CI: Fix Azure Pipelines problem with PATH
We should be able to set the environment variable `PATH` in an Azure Pipelines job with `echo '##vso[task.prependpath]$HOME/miniconda3/bin'`. See https://github.com/pandas-dev/pandas/blob/master/azure-pipelines.yml#L27

But that didn't work when we implemented it, so we ended up setting the path manually in every step (e.g. https://github.com/pandas-dev/pandas/blob/master/azure-pipelines.yml#L48).

This is adding noise to our CI configuration, making it harder to see what every step is doing. We should try to understand what's the problem with setting the `PATH`, and fix it.

This also applies to `ci/azure/posix.yml` (besides `azure-pipelines.yml` in the root).