Building on osx: Usage of tar --transform flag

[joprice]

When trying to build on a mac, it failed to due the missing --transform flag on the bsd tar the ships with mac. I ended up installing gtar and linking it in /usr/local. Perhaps these genrules can be rewritten to either not depend on that feature, or use a packaged version of tar?

[leostera]

Since we already have nix in place, perhaps we should just manage tar with it?