witness assignment of sha256 circuit has bug

[kunxian-xia]

What command(s) is the bug in?

RUST_LOG=debug cargo test --release --package zkevm-circuits --lib vk_stable -- --nocapture

Describe the bug

Some 1st phase column's value is assigned to be None which must not be the case.

Concrete steps to reproduce the bug. If it's able reproduce via testool, please share test_id from jenkins report

Use the branch bug/sha256 and then run the above cmd.

[noel2004]

Has been fixed:

It is caused by copy assigning cells from phase2 col into phase1 col. In the first pass such phase 1 cells would not be assigned and lead to a complain of 'Syntheis' from halo2.