MSBuild is not installed – I use MSVC 6.0 still. I would have discovered that was needed if I'd not been booted out of the CMD prompt ;-)

Ok, indeed if MSBuild.exe cannot be found, the exit would exit even the parent cmd console. It should be exit /B. Fixed:
Issue 2341 – Source code package build script exits parent cmd console on error
But you still didn't tell me, if the problem was indeed that MSBuild.exe was not installed. Was it?

I was just running it from a cmd window but it closed that each time – was very frustrating

Sorry for my late response.
I'm not sure I understand. Do you mean that despite the MSBuild.exe existing (and where exactly?), the batch file stops on that line? It does not for me.
The set part definitely should not be there. That's mistake:
Issue 2340 – Stray "set" command in source code package build script
But it still should not cause problems, as long as the MSBuild.exe is found.
Does it show any error? (run the batch from an existing cmd console window, to see it)

the line

causes the DOS box to close. I tried removing the set as it didn't seem appropriate but made no difference. Replacing the line with

and all is sweetness and light.
This is running on Windows 10 Pro BTW.
Let me know if any more info needed.