Since we now have SOURCEDIR_{FUCHSIA,AKAROS,NETBSD} exported in the
syz-big-env docker image, this will make CI fail for broken cross-builds too.
Update instructions in the docker image to fix the current problem
with permissions in syz-big-env: we need to tar with --mode=go=u.
On top of syz-env it provides akaros/fuchsia/netbsd toolchains and gcloud sdk.
With this it's possible to run dashboard/app tests on CI and locally
and test executor build and pkg/{csource,cover} for these OSes.
Update #1765
