[bug] The dotnet/runtime version retrieved for SDK perf scenarios belongs to dotnet/dotnet

The dotnet/runtime version retrieved for SDK perf scenarios stopped working since the deployment of VMR. https://github.com/dotnet/performance/blob/0cd6e3e4914c31ac05e24ffc658b0af2f6897fed/src/scenarios/shared/versionmanager.py#L40

The presented version now comes from dotnet/dotnet repo instead of dotnet/runtime repo. That is because in `get_sdk_versions` we are reading `VersionInfo` property from the used .dlls but since the dlls are now built as part of dotnet/dotnet they carry the version from there.

As a workaround, the actual dotnet/runtime commit can be retrieved by reading the `commitSha` from [https://github.com/dotnet/dotnet/blob/{dotnet/dotnet_hash}/src/source-manifest.json#L109-L113](https://github.com/dotnet/dotnet/blob/%7Bdotnet/dotnet_hash%7D/src/source-manifest.json#L109-L113).