Project ERROR: Cannot run compiler 'cl'. Maybe you forgot to setup the environment? Command exited with code 3

Old issues that was supposedly fixed in 5.9.1 in 

 "QMAKE_CXX.COMPILER_MACROS is not defined" with -tp vc

https://ci.appveyor.com/project/xyzz/acquisition/build/1.0.352

Now, in 5.9.1 there is a new error message I'm seeing:

https://ci.appveyor.com/project/xyzz/acquisition/build/1.0.358

That being:

Project ERROR: Cannot run compiler 'cl'. Maybe you forgot to setup the environment? Command exited with code 3

This problem is causing a basic build using Appveyor to fail.

Link to relevent Appveyor issue requesting update to 5.9.1 to work around the issue:

https://github.com/appveyor/ci/issues/1635

I am able to work around this problem in AppVeyor by adding a line to load vcvarsall.bat but that is something extra/unexpected that users now need to add to work around what was working without issue in QT 5.8:

 before_build:

• call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat" amd64
• C:\Qt\5.9.1\msvc2017_64\bin\qmake.exe -tp vc