Check DOTNET_HOST_* for host (COREHOST_*) tracing environment variables

[elinor-fung]

@am11 had a good suggestion of using the DOTNET_ prefix for COREHOST_ environment variables as a move toward greater consistency for .NET environment variables: #117797 (comment)

This updates the host to also check DOTNET_HOST_* for existing COREHOST_* variables used for tracing.

cc @dotnet/appmodel @AaronRobinsonMSFT @richlander

[dotnet-policy-service]

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[Copilot]

Pull Request Overview

This PR updates the host tracing environment variables from the legacy COREHOST_* prefix to the newer DOTNET_HOST_* prefix for better consistency with other .NET environment variables. The changes maintain backward compatibility by checking both prefixes with DOTNET_HOST_* taking precedence.

Key changes:

• Implements fallback logic to check both DOTNET_HOST_* and COREHOST_* environment variables
• Updates error messages and documentation to reference the new variable names
• Updates test constants and adds backward compatibility test coverage

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/native/corehost/hostpolicy/hostpolicy_context.cpp	Updates error message to reference DOTNET_HOST_TRACE instead of COREHOST_TRACE
src/native/corehost/hostmisc/trace.cpp	Implements fallback logic for environment variable checking and updates comments
src/installer/tests/TestUtils/Constants.cs	Updates test constants to use new DOTNET_HOST_* variable names
src/installer/tests/HostActivation.Tests/Tracing.cs	Refactors tests to be simpler and adds backward compatibility test

[elinor-fung]

/ba-g timeouts - for the one leg that had host tests, it timed out waiting for the Ubuntu run - the console logs show the tests all passed.