Install the .NET Core SDK into the repo root instead of UserProfile and 'install' copy of AspNetCore shared framework (#7293)

This is required to workaround several limitations in the way the .NET Core SDK finds shared frameworks and targeting packs. It allow tests to use shared frameworks and targeting packs.

It also matches the patterns established in other aspnet and dotnet repos. This should reduce the friction required to adopt Arcade SDK.

## Changes

* This moves the default location of the .NET Core SDK installation into `$repoRoot/.dotnet`. This location was already in use for CI builds. 
* Update the build step for Microsoft.AspNetCore.App to install the shared framework into the local copy of the .NET Core SDK

## Recommendations

* Use the "startvs.cmd" script to launch Visual Studio. This will set required environment variables to make VS happier than if you just double click the .sln file.
* Use "activate.sh/ps1" if you want to run `dotnet build`, `dotnet test` and other dotnet commands. These will set required environment variables, including PATH.
* I recommend removing %USERPROFILE%/.dotnet to your PATH variable if you had added it manually before. This will no longer match what build tools will install.
* `git clean -xfd -e .dotnet/` preserves the folder so you don’t have to re-download the SDK again.