Arcade OSX CI builds don't use local .NET Core SDK

https://dev.azure.com/dnceng/public/_build/results?buildId=115972

Typical Arcade Sdk builds are supposed to use the global .NET Core SDK (`[user profile]\.dotnet`), and Arcade Sdk **CI** builds are supposed to install a local .NET Core SDK (`[repo root]\.dotnet`).

From the same build pipeline, the Linux job is loading the local .NET Core SDK (`/__w/1/s/`)

```
Assembly = /__w/1/s/.dotnet/sdk/2.1.403/Sdks/NuGet.Build.Tasks.Pack/build/../CoreCLR/NuGet.Build.Tasks.Pack.dll
```

The OSX job is loading the user profile .NET Core SDK (`/Users/vsts/`)

```
Assembly = /Users/vsts/.dotnet/sdk/2.2.104/Sdks/NuGet.Build.Tasks.Pack/build/../CoreCLR/NuGet.Build.Tasks.Pack.dll
```

In the case of the arcade-minimalci-sample repo, it defines the local sdk as "2.1.403" whereas the sdk installed in the global cache of the machine is "2.2.104".

This is causing arcade-minimcalci-sample PR's to fail.

FYI @tmat @jonfortescue @MattGal