Update the code coverage action to only run on code files in src

[pmaytak]

We should update the code coverage action to only run on \src folder and only code files (.cs).

The Include section in the settings does include .dlls, so interesting that the analysis runs on everything else too. Maybe source files need to be specified instead of DLLs?

For example, here it runs on workflow file:
Add release tactics template by kellyyangsong · Pull Request #2941 · AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet