Utilize RestoreLockedMode

To properly utilize the NuGet lock files, we'll need to set the `RestoreLockedMode` key in our build system. See the details [here](https://devblogs.microsoft.com/nuget/enable-repeatable-package-restores-using-a-lock-file/).

Currently, other set of NuGet packages may be resolved for building platform-dependent packages; we may choose to enforce `--no-restore` on publish, and add the platform-dependent packages to the lock files.