fix env var for py38 install

appveyor.yml

@@ -1,51 +1,51 b''
 build: false
 matrix:
   fast_finish: true     # immediately finish build once one of the jobs fails.
 
 environment:
   matrix:
    - PYTHON: "C:\\Python36"
      PYTHON_VERSION: "3.6.x"
      PYTHON_ARCH: "32"
 
    - PYTHON: "C:\\Python36-x64"
      PYTHON_VERSION: "3.6.x"
      PYTHON_ARCH: "64"
 
    - PYTHON: "C:\\Python37"
      PYTHON_VERSION: "3.7.x"
      PYTHON_ARCH: "32"
 
    - PYTHON: "C:\\Python37-x64"
      PYTHON_VERSION: "3.7.x"
      PYTHON_ARCH: "64"
 
    - PYTHON: "C:\\Python38"
      PYTHON_VERSION: "3.8.x"
      PYTHON_ARCH: "32"
 
    - PYTHON: "C:\\Python38-x64"
      PYTHON_VERSION: "3.8.x"
      PYTHON_ARCH: "64"
 
 init:
   - "ECHO %PYTHON% %PYTHON_VERSION% %PYTHON_ARCH%"
 
 install:
   - ps: |
       Set-PSDebug -Trace 1
       if ($Env:PYTHON_VERSION.StartsWith("3.8")) {
         if ($Env:PYTHON_ARCH -eq "64") {
-          iex "choco install python3 --version=3.8 --no-progress --params /InstallDir:$Env:PYTHON_HOME"
+          iex "choco install python3 --version=3.8 --no-progress --params /InstallDir:$Env:PYTHON"
         } else {
-          iex "choco install python3 --version=3.8 --no-progress --params /InstallDir:$Env:PYTHON_HOME --x86"
+          iex "choco install python3 --version=3.8 --no-progress --params /InstallDir:$Env:PYTHON --x86"
         }
       }
   - "SET PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
   - "%CMD_IN_ENV%  python -m pip install --upgrade setuptools pip"
   - "%CMD_IN_ENV%  pip install nose coverage"
   - "%CMD_IN_ENV%  pip install .[test]"
   - "%CMD_IN_ENV%  mkdir results"
   - "%CMD_IN_ENV%  cd results"
 test_script:
   - "%CMD_IN_ENV%  iptest --coverage xml"