Net5.0 RC2: When building a solution with a target runtime ID an error is given

[zontreck]

In previous versions of dotnet core, you could build a solution with the runtime flag -r and it would build for that target (ex. dotnet build -c Release -r win-x64. Now however with RC2 it appears to echo out the error:

error NETSDK1134: Building a solution with a specific RuntimeIdentifier is not supported. If you would like to publish for a single RID, specifiy the RID at the individual project level instead. 

If this is the new normal, is there a replacement method for cross-compiling an entire solution for multiple platforms?