[ArPow] The asset manifest produced by the source-build leg should be renamed in terms of source-build

[MichaelSimons]

The asset manifest produced by the source-build leg is not explicitly named and gets the default name of Unix-AnuCpu.xml. This should be named in terms of source-build so that it is meaningful and doesn't have a chance of colliding with other manifests.

This was something caught by @hoyosjs in dotnet/diagnostics#2240.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.