fix(ghaction): Use SHA256SUMS filename instead of checksums.txt

[bsedat]

Fixes the generated GitHub Action script to properly reference the published SHA256SUMS file.

[roborev-ci]

roborev: Combined Review (5953ecd)

Verdict: No medium-or-higher issues found; the change looks clean.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

[wesm]

thanks!

[wesm]

Personally I have never use the GitHub Actions incarnation — I understand the desire but I prefer to retain all of the review data in a persistent database (the roborev-ci bot here runs on top of a dedicated roborev daemon instance that reviews all PRs on all my projects via the CI poller)