tools/docker/old-env: update to Go 1.16
tools/docker/old-env: update to Go 1.16

Our policy is to support 2 latest releases of Go.
Currently this is Go 1.16 and 1.17.
So update the old-env to the oldest release.
This will allow us to use lots of goodness available in the latest versions.

google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller