Fix fetch-latest-gax script to account for SDK differences
Fix fetch-latest-gax script to account for SDK differences

We need to run "dotnet pack" in the GAX working directory to pick up SDK 3.1, otherwise GAX doesn't build.

jskeet/gcloud-dotnet,googleapis/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet