Add testing of the release manifest file usage in the build script

[mthalman]

The VMR's build script takes the SB release manifest file as input: https://github.com/dotnet/dotnet/blob/92464d46540df2a24d896d56bf825a6ec67aa47f/build.sh#L36

There should be automated testing of this logic of the build script in order to avoid issues like #5337.