CI: correct azure-36-locale slow name + move pyarrow
Follow-up on https://github.com/pandas-dev/pandas/pull/30039

I put it in the wrong file, because the file names were swapped...

Now, here, I just renamed both files to the other name (the diff apparently cannot the rename, therefore you see a lot of changes, but I can ensure I didn't change anything except for pyarrow). 
But, I could also leave the files as is and switch them in the posix.yml file ? (use the envs for the different build)

I don't know if there was something specific in either environment that was specifically added for the slow tests?

cc @datapythonista 