Handle new DOTNET_ in complement to old COMPlus_ envvar prefix on Mono. (#56098)

Handle new DOTNET_ in complement to old COMPlus_ envvar prefix on Mono. (#56098)

https://github.com/dotnet/runtime/pull/50507 added support for DOTNET_
prefix for env variables previously using COMPlus_. That change was
however not implemented on Mono, still using COMPlus for some EventPipe,
DiagnosticServer env variables. This fix adds similar support on Mono,
first looking for a DOTNET_ version of the environment variable and if
not found, fallback to old COMPlus_ as fallback.

Documentation for .net6 have been updated to use new names:
https://docs.microsoft.com/en-us/dotnet/core/diagnostics/eventpipe#trace-using-environment-variables