Yet another attempt

Author: zpostfacto

.github/setup-msvc-env.bat

@@ -1,9 +1,14 @@
 @echo off
 rem Locate the latest Visual Studio installation that has the VC++ toolchain,
-rem then call VsDevCmd.bat to put cl.exe, link.exe, ninja, etc. on PATH.
+rem then call vcvarsall.bat to put cl.exe, link.exe, ninja, etc. on PATH.
 rem
 rem Usage: call .github\setup-msvc-env.bat
 rem Must be invoked with CALL so environment changes propagate back to the caller.
+rem
+rem NOTE: We use vcvarsall.bat (not VsDevCmd.bat) because VsDevCmd.bat is designed
+rem for interactive developer shells and can silently skip toolchain PATH setup
+rem when called non-interactively from within a script. vcvarsall.bat is the
+rem documented and supported way to initialise the MSVC toolchain from automation.
 
 for /f "usebackq delims=" %%i in (
   `"%ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe" ^
@@ -17,7 +22,7 @@ if "%VSINSTALLDIR%"=="" (
     exit /b 1
 )
 
-call "%VSINSTALLDIR%\Common7\Tools\VsDevCmd.bat" -arch=amd64 -host_arch=amd64
+call "%VSINSTALLDIR%\VC\Auxiliary\Build\vcvarsall.bat" x64
 
 rem Prevent inherited GCC/Clang hints from steering CMake to MinGW.
 set "CC="