pkg/gcs: remove support for gs:// prefixed paths
pkg/gcs: remove support for gs:// prefixed paths

The problem with gs:// is that after any filepath.Join
it's transformed to gs:/ which is not recognized.
It's safer to not allow specifying them at all,
rather than constantly breaking them.

dotweiba/syzkaller,dotweiba/syzkaller,google/syzkaller,google/syzkaller,hyz0906/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,zoulasc/syzkaller,xairy/syzkaller,xairy/syzkaller,zoulasc/syzkaller,google/syzkaller,hyz0906/syzkaller,xairy/syzkaller,hyz0906/syzkaller,google/syzkaller,zoulasc/syzkaller,xairy/syzkaller,google/syzkaller,dotweiba/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,hyz0906/syzkaller,hyz0906/syzkaller,google/syzkaller,xairy/syzkaller