Add ftpmirror.gnu.org as primary and add fallback for bison download

[gigeresk]

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[greptile-apps]

Greptile Summary

This follow-up updates the Bison download fallback so both mirrors write to the same explicit output path, ensuring the fallback replaces any partial primary download.

• Uses ftpmirror.gnu.org as the primary Bison mirror.
• Retains mirrors.kernel.org as the fallback.
• Fixes the previously reported numbered-filename behavior by supplying -O to both wget commands.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the fallback writes directly over any partial primary archive, and extraction runs only after a download command succeeds.

Important Files Changed

Filename	Overview
Dockerfile.centos7	The fallback now truncates and replaces the primary output file before extraction, resolving the prior partial-archive issue.

_{Reviews (2): Last reviewed commit: "Account for partial downloads" | Re-trigger Greptile}

[gigeresk]

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