Fix VS15 toolchain configure: hide output from VsDevCmd.bat
VsDevCmd.bat outputs header
```
**********************************************************************
** Visual Studio 2019 Developer Command Prompt v16.3.10
** Copyright (c) 2019 Microsoft Corporation
**********************************************************************
```
which interferes with
```
echo %VCINSTALLDIR%
```
and doesn't allow to configure VS toolchain.
Closes
@@ -157,7 +157,7 @@ def find_vc_path(repository_ctx):
repository_ctx.file(
"get_vc_dir.bat",
"@echo off\n" +
- "call \"" + script + "\"\n" +
+ "call \"" + script + "\" > NUL\n" +
"echo %VCINSTALLDIR%",
True,
)
