.cirrus.yml: Update envvars to match choco py38
.cirrus.yml: Update envvars to match choco py38

choco package python has been updated to version 3.8
As PyScaffold passes CI for Python 3.8 on Linux,
go with flow and use Python 3.8 on Windows also.

blue-yonder/pyscaffold,blue-yonder/pyscaffold