Lock ops based on computed key rather than LLB digest

[mikejholly]

In some cases, the LLB vertex digest is not a reliable way to disambiguate operations. This is leading to duplicated execs.

Example:

VERSION 0.8

PROJECT earthly-technologies/core

test:
  FROM alpine
  RUN echo "before copy"
  COPY input .
  RUN echo "after copy"

Before:

After:

earthly/earthly#4130

[alexcb]

looks good, but please squash your commits.