Improve preview handling for dotnet workload search version

[dsplaisted]

Currently, when searching for workload sets, the SDK will include preview versions if the .NET SDK itself is branded as preview:

sdk/src/Cli/dotnet/commands/dotnet-workload/search/WorkloadSearchVersionsCommand.cs

Line 79 in 54abeef

versions = PackageDownloader.GetLatestPackageVersions(packageId, _numberOfWorkloadSetsToTake, packageSourceLocation: null, includePreview: !string.IsNullOrWhiteSpace(_sdkVersion.Prerelease))

Probably there should be a command-line parameter that controls this (ie --include-previews).

Additionally, checking _sdkVersion.Prerelease may not be the best option for the default behavior. A better option may be to base it on whether the SDK Feature band is a prerelease band. That way the behavior of stabilized and non-stabilized RTM-branded SDKs would be the same, where today it isn't.

Finally, currently if you run dotnet workload search version --include-previews, you get an IndexOutOfRangeException, instead of an error that --include-previews is unrecognized.