Discourage building with MSBUILDDEBUGONSTART (#4952)

Discourage building with MSBUILDDEBUGONSTART. Allow debugging of a build if MSBUILDDEBUGONSTART_HARD is defined.

Author: Forgind

build.cmd

@@ -1,3 +1,10 @@
 @echo off
+setlocal
+if defined MSBUILDDEBUGONSTART_HARD goto build
+if not defined MSBUILDDEBUGONSTART goto build
+if %MSBUILDDEBUGONSTART% == 0 goto build
+set MSBUILDDEBUGONSTART=
+echo To debug the build, define a value for MSBUILDDEBUGONSTART_HARD.
+:build
 powershell -NoLogo -NoProfile -ExecutionPolicy ByPass -Command "& """%~dp0eng\common\build.ps1""" -build -restore %*"
 exit /b %ErrorLevel%