Debugger fails when using "externalTerminal" option on Linux.

## Environment data
`dotnet --info` output:
```.NET Core SDK (reflecting any global.json):
Version: 2.1.403
Commit: 04e15494b6

Runtime Environment:
OS Name: arch
OS Version:
OS Platform: Linux
RID: arch-x64
Base Path: /opt/dotnet/sdk/2.1.403/

Host (useful for support):
Version: 2.1.5
Commit: 290303f510

.NET Core SDKs installed:
2.1.403 [/opt/dotnet/sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.5 [/opt/dotnet/shared/Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.5 [/opt/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.5 [/opt/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download
```

VS Code version: 1.28.2
C# Extension version: 1.17.0

## Steps to reproduce
Create new Hello World console application using command:
```
dotnet new console
```
then change the console option to "externalTerminal" and build/Run the application.

## Expected behavior
Print "Hello World!" in an external terminal.

## Actual behavior
External terminal shows up, program looks like it hangs while debugging and nothing prints. Exit out of the external terminal and VSCode prompts you with an alert box saying open launch.json.