_NT_SYMBOL_PATH environment is not appreciated by the debugger

I think it is bug, although I am not sure if it is supported feature.

* Operating System and version: Windows 7
* VS Code version and if you are using the Insiders build: 1.17.2 (not Insiders)
* C/C++ extension version: 0.14.2
* Other extensions you installed and if the issue persists after disabling them: not sure
* step-by-step instructions to reproduce the issue: 

I am launching a binary under the debugger with the following settings:

     {
            "name": "(Windows) Launch",
            "type": "cppvsdbg",
            "request": "launch",
            "program": "${workspaceFolder}/target/debug/hello_cargo.exe",
            "args": [],
            "stopAtEntry": false,
            "cwd": "${workspaceFolder}",
            "environment": [],
            "externalConsole": true
        }

A binary is a sample rust program compiled to native MSVC.

The debugger shows the output:

-------------------------------------------------------------------
You may only use the C/C++ Extension for Visual Studio Code with
Visual Studio Code, Visual Studio or Visual Studio for Mac software
to help you develop and test your applications.
-------------------------------------------------------------------
Loaded 'C:\Projects\Tait\hello_cargo\target\debug\hello_cargo.exe'. Symbols loaded.
Loaded 'C:\Windows\System32\ntdll.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\kernel32.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\KernelBase.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\advapi32.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\msvcrt.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\sechost.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\rpcrt4.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\ws2_32.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\nsi.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\vcruntime140.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\api-ms-win-crt-runtime-l1-1-0.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\ucrtbase.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\api-ms-win-core-timezone-l1-1-0.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\api-ms-win-core-file-l2-1-0.dll'. Cannot find or open the PDB file.
Loaded 'C:\Windows\System32\api-ms-win-core-localization-l1-2-0.dll'. Cannot find or open the PDB file.

I can set a breakpoint in my code and catch it successfully, but frames from ntdll, kernel32, etc. are not resolved. Although, I have got symbols path set:

_NT_SYMBOL_PATH=srv*C:\symbols*http://msdl.microsoft.com/downloads/symbols

Local symbols cache folder is not getting populated.

Expected behavior:

• PDB files are getting down loaded, cached locally, and loaded by the debugger.

Start by reproducing the Windows 7 launch configuration with the cppvsdbg debugger and the _NT_SYMBOL_PATH value shown in the issue. Check how the VS Code C/C++ extension handles that environment variable when launching the Rust/MSVC binary. Done means Windows system PDB files are downloaded to the configured cache and their symbols are loaded for debugger frames.