The `DISABLE_GITHUB_REPORTER` environment variable should be named `TUNIT_DISABLE_GITHUB_REPORTER`

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It's always a good idea to prefix environment variables to reduce potential conflicts. The intent is also clearer with the TUNIT_ prefix.

• GitHub environment variables are all prefixed with GITHUB_
• .NET environment variables are all prefixed with DOTNET_
• go environment variables are all prefixed with GO
• etc.

Maybe both TUNIT_DISABLE_GITHUB_REPORTER and DISABLE_GITHUB_REPORTER could be supported for some time in order to avoid breaking existing workflows.