.Net: Fix bash script to build the dotnet solution

[jchomarat]

Motivation and Context

Small fix for the build.sh bash in the dotnet folder to locally build and test the solution. It was not working as the script was already in the dotnet folder

Description

Provide another way to navigate to the dotnet folder before triggering dotnet build & dotnet test commands.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄