Work around apparent bug in CMD's FOR /L

Hopefully this will mean that official build failures during the actual build
phase will correctly be reported as failures.  There could be other places that
need to change, but this is all we know of right now.

Fixes #18788 (at least part of it)

build-test.cmd

@@ -343,7 +343,10 @@ for /l %%G in (1, 1, %__BuildLoopCount%) do (
         echo     %__BuildLog%
         echo     %__BuildWrn%
         echo     %__BuildErr%
-        exit /b 1
+        REM This is necessary because of a(n apparent) bug in the FOR /L command.  Under certain circumstances,
+        REM such as when this script is invoke with CMD /C "build-test.cmd", a non-zero exit directly from
+        REM within the loop body will not propagate to the caller.  For some reason, goto works around it.
+        goto     :Exit_Failure
     )
 
     set __SkipPackageRestore=true
@@ -633,3 +636,6 @@ ren "%CORE_ROOT%\temp.ni.dll" %2
 
 echo Successfully precompiled %2
 exit /b 0
+
+:Exit_Failure
+exit /b 1