ci(release): drop GHA build cache from docker push (unblock releases)

[ch4r10t33r]

Problem

The devnet5 release workflow has failed the last two releases (v0.5.8 and v0.5.9) at the docker push step:

#16 ERROR: failed to push docker.io/.../zeam:0.5.9-amd64: ... error writing layer blob: failed to reserve cache
ERROR: failed to build: failed to solve: error writing layer blob: failed to reserve cache

The tags get created but no image is published (create-manifest and create-github-release are then skipped).

Root cause

The Build and push Docker image step in auto-release.yml uses:

cache-from: type=gha
cache-to: type=gha,mode=max

failed to reserve cache is the GitHub Actions cache backend rejecting a reserveCache call. mode=max exports every layer of the large zeam image (the ~240 MB prover staticlib plus rust deps), and both matrix legs (amd64 + arm64) write to the same cache at once, exhausting the repo's 10 GB GHA cache quota. When the cache export fails, buildkit aborts the whole build-and-push solve, so the image never pushes. It is not transient flakiness: it fires at the same step on every run.

Fix

Remove the GHA build cache from the release push. Dockerfile.prebuilt only packages an already-built binary, so the layer cache saves almost nothing here, and a release must never be blocked by a best-effort build cache.

Follow-up (completing v0.5.9)

The v0.5.9 / Devnet5 git tags already exist from the failed run, and create-tags hard-fails if a version tag already exists. So after this merges, the stuck v0.5.9 release needs to be re-driven with the fixed workflow, either by deleting the dangling v0.5.9/Devnet5 tags and re-opening the release PR, or by cutting it as a new version label. I can do that once this is in.