Add dotnet env variable to workflow

[angelapwen]

Previously, we considered go a non-traced language, and so the tracer was not imported to this workflow at all. Now that we are considering it traced, we need to add this additional environment variable, which is required for our MacOS tests to not time out waiting for a GUI popup.

We already do this in several other Go-specific checks, such as https://github.com/github/codeql-action/blob/main/.github/workflows/__go-custom-tracing-autobuild.yml and https://github.com/github/codeql-action/blob/main/.github/workflows/__go-reconciled-tracing-autobuilder.yml

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.