[BUILD] Fix first time DotNetUtilities build failing
[BUILD] Fix first time DotNetUtilities build failing

Use dotnet restore flag to avoid NETSDK1004 error that would occur
at first time when building solution.
The restore process only rebuilds what's not yet rebuilt so it doesn't
slow down the process.
https://docs.microsoft.com/en-us/dotnet/core/tools/sdk-errors/netsdk1004

dontnod/nimp