build: prevent VsDevCmd.bat from changing cwd

VsDevCmd.bat changes the current working directory to
"%USERPROFILE%\Source" if it exists. Setting VSCMD_START_DIR overrides
this behavior.

PR-URL: #14303
Fixes: #14300
Refs: https://developercommunity.visualstudio.com/content/problem/26780/vsdevcmdbat-changes-the-current-working-directory.html
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

vcbuild.bat

@@ -176,6 +176,8 @@ if "_%VisualStudioVersion%_" == "_15.0_" if "_%VSCMD_ARG_TGT_ARCH%_"=="_%target_
 set "VSINSTALLDIR="
 call tools\msvs\vswhere_usability_wrapper.cmd
 if "_%VCINSTALLDIR%_" == "__" goto vs-set-2015
+@rem prevent VsDevCmd.bat from changing the current working directory
+set "VSCMD_START_DIR=%CD%"
 set vcvars_call="%VCINSTALLDIR%\Auxiliary\Build\vcvarsall.bat" %vcvarsall_arg%
 echo calling: %vcvars_call%
 call %vcvars_call%