Avoid sending sources to offload servers

[Simen Endsjø]

I notice package sources are sent to offload servers. I have a build that has
1.9GB sources, and results in a 4.7GB store item. I would have thought only the
definitions were sent to the build machine, and it would download the 1.9GB
file, but it seems this file is sent from my machine. I cannot see a way around
this, and it makes it difficult to combine this package with packages I wish to
offload as I cannot state certain packages shouldn't be offloaded.

Not sure what I'm asking here, just wanted to point out an issue which causes
problems.