[azdo] add initSteps for installing .NET 6 (#1082)
[azdo] add initSteps for installing .NET 6 (#1082)

Context: https://github.com/xamarin/AndroidX/pull/247

To improve upon the changes in abda7267, we could add a new
`initSteps` to install .NET 6.

Previously, if we could only install .NET 6 during `preBuildSteps`,
then the `dotnet tool` commands would fail:

    - pwsh: |
        dotnet tool install -g api-tools --version ${{ parameters.apiTools }}
    ...
    displayName: 'Install required .NET Core global tools'

Because .NET 6 is defined in `global.json`, you would get an error
saying there was no .NET 6 installed.

If we add a new `initSteps`, we could do something like:

    initSteps:
      - pwsh: |
          $ProgressPreference = 'SilentlyContinue'
          Invoke-WebRequest -Uri "https://dot.net/v1/dotnet-install.ps1" -OutFile dotnet-install.ps1
          & .\dotnet-install.ps1 -Version $(DotNet6Version) -InstallDir "$env:ProgramFiles\dotnet\" -Verbose
    tools:
      - 'xamarin.androidbinderator.tool': '0.4.2'
      - 'xamarin.androidx.migration.tool': '1.0.7.1'

Then the `dotnet tool` commands will succeed.
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