Rename sigOps to sigCheks in the miner

Summary:
Similar to D12783, but for the block assembler.

Depends on D12783.

Test Plan:

ninja check-extended
ninja bitcoin-fuzzer.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12785