build,win: skip `vcvarsall.bat` if env is set

PR-URL: https://github.com/nodejs/node/pull/13806
Fixes: https://github.com/nodejs/node/issues/13765
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Kunal Pathak <kunal.pathak@microsoft.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

vcbuild.bat

@@ -200,9 +200,12 @@ if defined msi (
     goto wix-not-found
   )
 )
-@rem VS2015 vsvarsall is quick, so run anyway
+
+@rem check if VS2015 is already setup
+if "_%VisualStudioVersion%_" == "_14.0_" if "_%VCVARS_VER%_" == "_140_" goto found_vs2015
 call "%VS140COMNTOOLS%\..\..\vc\vcvarsall.bat"
 SET VCVARS_VER=140
+:found_vs2015
 if not defined VCINSTALLDIR goto msbuild-not-found
 @rem Visual C++ Build Tools 2015 does not define VisualStudioVersion
 echo Found MSVS version 14.0