Failed download source code file when debugging .

## Environment data
`dotnet --info` output:
```
.NET SDK (reflecting any global.json):
Version: 5.0.403
Commit: bf9d168ac2

Runtime Environment:
OS Name: ubuntu
OS Version: 20.04
OS Platform: Linux
RID: ubuntu.20.04-x64
Base Path: /usr/share/dotnet/sdk/5.0.403/

Host (useful for support):
Version: 5.0.12
Commit: 7211aa01b3

.NET SDKs installed:
5.0.403 [/usr/share/dotnet/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 5.0.12 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 5.0.12 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET runtimes or SDKs:
https://aka.ms/dotnet-download
```
VS Code version:
C# Extension version:
```
v1.24.4
```
## OmniSharp log
```
Starting OmniSharp server at 5/16/2022, 1:55:48 AM
Target: /home/qishuo/repos/PocketWeb/Pocket.sln

OmniSharp server started.
Path: /home/qishuo/.vscode-server/extensions/ms-dotnettools.csharp-1.24.4-linux-x64/.omnisharp/1.38.2/run
PID: 25024

[info]: OmniSharp.Stdio.Host
Starting OmniSharp on ubuntu 20.4 (x64)
[info]: OmniSharp.Services.DotNetCliService
Checking the 'DOTNET_ROOT' environment variable to find a .NET SDK
[info]: OmniSharp.Services.DotNetCliService
Using the 'dotnet' on the PATH.
[info]: OmniSharp.Services.DotNetCliService
DotNetPath set to dotnet
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
Located 1 MSBuild instance(s)
1: StandAlone 17.1.1 - "/home/qishuo/.vscode-server/extensions/ms-dotnettools.csharp-1.24.4-linux-x64/.omnisharp/1.38.2/omnisharp/.msbuild/Current/Bin"
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
MSBUILD_EXE_PATH environment variable set to '/home/qishuo/.vscode-server/extensions/ms-dotnettools.csharp-1.24.4-linux-x64/.omnisharp/1.38.2/omnisharp/.msbuild/Current/Bin/MSBuild.exe'
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
Registered MSBuild instance: StandAlone 17.1.1 - "/home/qishuo/.vscode-server/extensions/ms-dotnettools.csharp-1.24.4-linux-x64/.omnisharp/1.38.2/omnisharp/.msbuild/Current/Bin"
CscToolExe = csc.exe
MSBuildToolsPath = /home/qishuo/.vscode-server/extensions/ms-dotnettools.csharp-1.24.4-linux-x64/.omnisharp/1.38.2/omnisharp/.msbuild/Current/Bin
CscToolPath = /home/qishuo/.vscode-server/extensions/ms-dotnettools.csharp-1.24.4-linux-x64/.omnisharp/1.38.2/omnisharp/.msbuild/Current/Bin/Roslyn
BypassFrameworkInstallChecks = true
MSBuildExtensionsPath = /home/qishuo/.vscode-server/extensions/ms-dotnettools.csharp-1.24.4-linux-x64/.omnisharp/1.38.2/omnisharp/.msbuild
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.CSharpFormattingWorkspaceOptionsProvider, Order: 0
[info]: OmniSharp.Cake.CakeProjectSystem
Detecting Cake files in '/home/qishuo/repos/PocketWeb'.
[info]: OmniSharp.Cake.CakeProjectSystem
Did not find any Cake files
[info]: OmniSharp.MSBuild.ProjectSystem
Detecting projects in '/home/qishuo/repos/PocketWeb/Pocket.sln'.
[info]: OmniSharp.MSBuild.ProjectManager
Queue project update for '/home/qishuo/repos/PocketWeb/Web/Web.csproj'
[info]: OmniSharp.Script.ScriptProjectSystem
Detecting CSX files in '/home/qishuo/repos/PocketWeb'.
[info]: OmniSharp.Script.ScriptProjectSystem
Did not find any CSX files
[info]: OmniSharp.WorkspaceInitializer
Configuration finished.
[info]: OmniSharp.Stdio.Host
Omnisharp server running using Stdio at location '/home/qishuo/repos/PocketWeb' on host 24933.
[info]: OmniSharp.MSBuild.ProjectManager
Loading project: /home/qishuo/repos/PocketWeb/Web/Web.csproj
[info]: OmniSharp.MSBuild.ProjectManager
Successfully loaded project file '/home/qishuo/repos/PocketWeb/Web/Web.csproj'.
[info]: OmniSharp.MSBuild.ProjectManager
Adding project '/home/qishuo/repos/PocketWeb/Web/Web.csproj'
[info]: OmniSharp.MSBuild.ProjectManager
Update project: Web
Received response for /v2/getcodeactions but could not find request.
Received response for /quickinfo but could not find request.
```
## Steps to reproduce

1. flow this [Debugging-into-the-.NET-Framework-itself](https://github.com/OmniSharp/omnisharp-vscode/wiki/Debugging-into-the-.NET-Framework-itself) to config lauch.json.
2. press F5 to lauch debug my asp.net core project.
3. set breakpoint and step into it.
4. See error

## Expected behavior

`the source file of List.cs`

## Actual behavior
```
// 在此计算机上找不到源文件
// /_/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs。此源文件模块的
// PDB 包含要通过源链接下载源文件的信息，但调试器请求下载此源文件失败。
//
// 请求失败: Connection refused (raw.githubusercontent.com:443) Connection refused
//
// 将从中下载源文件的 URL:
// https://raw.githubusercontent.com/dotnet/runtime/7211aa01b34bb55ca67bdddd6e80ce23ee201bd2/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs
//
// 有关源链接和 VS Code 的详细信息，请参阅: https://aka.ms/VSCode-DotNet-SourceLink
```

## Additional context

this is absolutely a network issue. Because the browser also can't open this link without proxy.

> https://raw.githubusercontent.com/dotnet/runtime/7211aa01b34bb55ca67bdddd6e80ce23ee201bd2/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs

how to make omnisharp-vscode download source code through proxy.