not all operations that involve workers need to build. for example nix
copy --from calls instantiate thir worker with the *origin* store, not
the store they copy *into*. for binary cache stores this is not likely
to ever work if cgroups are enabled and the copy is run on normal user
credentials, even though no cgroups will ever be needed to copy things
fixes#1088
Change-Id: I065e29e1a1d0f58d81823609ef0701ff82cdd1d5
eldritch horrors
c040a9d8e4