[2.7] bpo-30450: Fall back on the old env.bat (GH-3443)

diff --git a/PCbuild/build.bat b/PCbuild/build.bat
index 13da4be..b263ec7 100644
--- a/PCbuild/build.bat
+++ b/PCbuild/build.bat
@@ -101,7 +101,7 @@
 
 rem Setup the environment
 call "%dir%find_msbuild.bat" %MSBUILD%
-if ERRORLEVEL 1 (echo Cannot locate MSBuild.exe on PATH or as MSBUILD variable & exit /b 2)
+if ERRORLEVEL 1 (call "%dir%env.bat" && set MSBUILD=msbuild)
 
 if "%kill%"=="true" call :Kill