mz960: don't squash a base stage that declares ONBUILD

[mzihlmann]

Fixes #960

Squashing filters the base stage's ONBUILD declaration out of the merged command list. That is fine for the squash in MakeKanikoStages, which runs before any cache key exists, but the stage elimination behind FF_KANIKO_SKIP_CACHED_STAGES squashes after the lookahead has already hashed the unsquashed chain. The merged chain then hashes one command fewer, so every key after the ONBUILD diverges from the precomputed one. On a cache-consume build the inferred cross-stage key misses, the COPY --from falls back to hashing its source files, and the build aborts because the stage it reads from was eliminated and never wrote /kaniko/deps/N. So we leave a base stage that declares an ONBUILD unsquashed, it stays alive and writes its deps like it does with the flag off.

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📥 Commits

Reviewing files that changed from the base of the PR and between 93be5ff and 687c2ab.

📒 Files selected for processing (3)

• integration/dockerfiles/Dockerfile_test_issue_mz960
• integration/images.go
• pkg/executor/build.go

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