Missing nuget.org package source on BYOC AWS Linux

[chmely]

It seems that when I build our own AWS build image based on Linux the standard nuget.org package source is not present in ~/.nuget/NuGet/NuGet.Config when build starts

Tested scenarios:

1. BYOC Windows: (works as expected)

nuget:
  disable_publish_on_pr: true
  account_feed: true
  project_feed: true
  
install:
- cmd: |
    nuget
    nuget sources
    type %appdata%\NuGet\NuGet.Config

output:

NuGet Version: 5.11.0.10

Registered Sources:
  1.  nuget.org [Enabled]
      https://api.nuget.org/v3/index.json
  2.  AppveyorAccountFeed [Enabled]
      https://ci.appveyor.com/nuget/abcde-323ubderfg349
  3.  AppveyorProjectFeed [Enabled]
      https://ci.appveyor.com/nuget/abcde-m5sdafsdfslmw
  4.  Microsoft Visual Studio Offline Packages [Enabled]
      C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
    <add key="AppveyorAccountFeed" value="https://ci.appveyor.com/nuget/abcde-22348sdg8wh3" />
    <add key="AppveyorProjectFeed" value="https://ci.appveyor.com/nuget/abcde-m5sdafsdfslmw" />
  </packageSources>
  <packageSourceCredentials>
    <AppveyorAccountFeed>
      <add key="Username" value="nuget" />
      <add key="ClearTextPassword" value="6feac921a39" />
    </AppveyorAccountFeed>
    <AppveyorProjectFeed>
      <add key="Username" value="nuget" />
      <add key="ClearTextPassword" value="6feac921a39" />
    </AppveyorProjectFeed>
  </packageSourceCredentials>
</configuration>

2. Standard Linux Ubuntu image: (works as expected)

nuget:
  disable_publish_on_pr: true
  account_feed: true
  project_feed: true

install:
- sh: >-

    nuget

    cat ~/.nuget/NuGet/NuGet.Config

output:

NuGet Version: 6.4.0.123

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
    <add key="AppveyorAccountFeed" value="https://ci.appveyor.com/nuget/abcde-2dfdf5435sdfg" />
    <add key="AppveyorProjectFeed" value="https://ci.appveyor.com/nuget/abcde-m5sdafsdfslmw" />
  </packageSources>
  <packageSourceCredentials>
    <AppveyorAccountFeed>
      <add key="Username" value="nuget" />
      <add key="ClearTextPassword" value="291e42dd2" />
    </AppveyorAccountFeed>
    <AppveyorProjectFeed>
      <add key="Username" value="nuget" />
      <add key="ClearTextPassword" value="291e42dd66cd" />
    </AppveyorProjectFeed>
  </packageSourceCredentials>
</configuration>

3. BYOC Linux: (nuget.org feed missing)

nuget:
  disable_publish_on_pr: true
  account_feed: true
  project_feed: true

install:
- sh: >-

    dotnet nuget

    dotnet nuget list source

    cat ~/.nuget/NuGet/NuGet.Config

output:

NuGet Command Line 6.3.2.9

Registered Sources:
  1.  AppveyorAccountFeed [Enabled]
      https://ci.appveyor.com/nuget/abcde-2qsadsasdasda
  2.  AppveyorProjectFeed [Enabled]
      https://ci.appveyor.com/nuget/abcde-m5sdafsdfslmw


<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="AppveyorAccountFeed" value="https://ci.appveyor.com/nuget/abcde-2qsadsasdasda" />
    <add key="AppveyorProjectFeed" value="https://ci.appveyor.com/nuget/abcde-m5sdafsdfslmw" />
  </packageSources>
  <packageSourceCredentials>
    <AppveyorAccountFeed>
      <add key="Username" value="nuget" />
      <add key="ClearTextPassword" value="eb3b2sfd" />
    </AppveyorAccountFeed>
    <AppveyorProjectFeed>
      <add key="Username" value="nuget" />
      <add key="ClearTextPassword" value="eb3b2659" />
    </AppveyorProjectFeed>
  </packageSourceCredentials>
</configuration>

as you can see both the standard Linux and the Windows BYOC images do add the nuget.org source as expected. However when running our build on Linux BYOC that source is missing and only AppveyorAccountFeed and AppveyorProjectFeed are added automatically