cmd/cue/cmd: use tools/flow

This fixes various issues.

Fixes #584
Fixes #335
Fixes #281

Change-Id: I31e88cf121665d410cd5c64d0693ecca5c92e960
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7643
Reviewed-by: CUE cueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl