/sdk/core/Azure.Core/generate_solution.ps1 should create the sln file only in the `/sdk/core/Azure.Core` directory

[generate_solution.ps1](https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/core/Azure.Core/generate_solution.ps1) should ensure it creates the sln file only in the `/sdk/core/Azure.Core` directory