azure-sdk-for-net CI is building the entire service directory instead of targeting csproj files

https://dev.azure.com/azure-sdk/public/_build/results?buildId=3557397&view=logs&j=3dc8fd7e-4368-5a92-293e-d53cefc8c4b3&t=45862c6e-83c5-515f-73e2-e7009eff9f9b&l=20

This means that mgmt plane can fail for data plane reasons and vice versa.

It would be nice if we could isolate and build at the csproj level.

Start with the CI configuration responsible for the linked Azure DevOps build log and trace how the service directory is selected for builds. Confirm completion when management-plane and data-plane builds target only their intended csproj files and can no longer fail because of the other plane.

Written by the indexing model from the issue text.