Add support for DOTNET prefix on ICorProfiler env vars

[noahfalk]

The ICorProfiler feature currently uses environment variables such as CORECLR_PROFILER and other CORECLR_xyz names to activate a profiler at startup. As part of consolidating our environment variables to use the DOTNET prefix, we should support DOTNET_* naming on the profiler env vars.

I expect we'd want to start by adding DOTNET_ variations to the existing env vars, then sometime in the future (probably at least 3 years) we could remove support for the current CORECLR prefixed variables.

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @dotnet/dotnet-diag
See info in area-owners.md if you want to be subscribed.