- back port of #36824 and #37077

Get `dotnet` cores on Linux / macOS (#36824)

  - follow up to 0d0103b4
  - `COMPlus*` variables were uppercased and `dotnet` ignored them on Linux / macOS
  - only upload dumps in `failed()` jobs
    - use scripts to avoid more YAML complications and avoid empty artifacts

Remove unused parameter and add requested comments (#37077)

  - `parameters.buildScript` was only ever set to its default value
  - add comments explaining the `parameters.steps` expansion
    - see https://github.com/dotnet/aspnetcore/pull/36824#pullrequestreview-764795624