feat(ci): fill a zkevm variant tarball from its project branch

[spencer-tb]

Description

Stacked on #41: every tests@ and <feat>-devnet@ release also attaches <tarball>_zkevm.tar.gz — witness-enabled blockchain fixtures, filled from the projects/zkevm branch on its own runner through the standard build/combine pipeline.

Adds an optional ref to release variants: a variant with a ref fills from that branch's tip instead of the tagged commit, so zkevm fixtures ship with every tag without any zkevm code merging into this branch. The ref is the interim shape until a project's filling machinery is upstreamed — its variant then drops the ref and fills from the tagged commit; until then the tarball's content tracks the branch, not the tag.

A failed variant fill never blocks the release (per #41's variant mechanism).

Related Issues or PRs

Stacked on #41.

Checklist

• Ran fast static checks to avoid CI fails, see Code Standards & Verifying Changes: just static
• PR title has the form <type>(<area>): <title>, where <type> and <area> come from an appropriate C-<type>, respectively A-<area>, label. The title should match the target squash commit message.

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