build: call setlocal in vcbuild.bat

Currently the variables set in vcbuild.bat are mostly global and
escape/leak out into the calling process. For example, running
vcbuild.bat test and then echoing the config variable gives:

vcbuild.bat test
...
echo %config%
Release

After this change the same command give:
vcbuild.bat test
...
echo %config%
%config%

PR-URL: #15754
Reviewed-By: James M Snell <jasnell@gmail.com>

vcbuild.bat

@@ -1,5 +1,7 @@
 @echo off
 
+setlocal EnableExtensions
+
 cd %~dp0
 
 if /i "%1"=="help" goto help