lowercase the "path" command

Co-authored-by: Andrew Murray <[email protected]>

.appveyor.yml

@@ -53,7 +53,7 @@ test_script:
 - cd c:\pillow
 - '%PYTHON%\%EXECUTABLE% -m pip install pytest pytest-cov pytest-timeout defusedxml ipython numpy olefile pyroma'
 - c:\"Program Files (x86)"\"Windows Kits"\10\Debuggers\x86\gflags.exe /p /enable %PYTHON%\%EXECUTABLE%
-- PATH %PYTHON%;%PATH%
+- path %PYTHON%;%PATH%
 - .ci\test.cmd
 
 after_test: