ci: clear the build cache before retrying a bake

[viceice]

Changes

The bake action now runs docker buildx prune --force before each retry. An aborted build can leave a broken snapshot in the builder cache, and every retry then fails with failed to commit … snapshot … does not exist: not found instead of retrying the actual build (seen in the node test of #7662, where the first attempt failed on a transient download error). Errors from the prune only show up as warnings, so it can't make a retry fail.

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Written by Claude Opus 5.5 in Claude Code.

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actionlint and zizmor pass. The retry path only runs after a failed build, so CI here doesn't exercise it.

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