Don't zip toolchain/go/update_prebuilts.sh

Putting update_prebuilts.sh into into go.zip causes it to get copied
to prebuilts/go/linux-x86, where it doesn't work because the relative
path to the top of the tree is wrong.

Test: treehugger
Change-Id: I7c507db65fe1cc5e1b38f2750ae583fab9f08b85