chore: update golangci-lint to v2.12.2 (backport #565)

[mergify]

longhorn-spdk-engine was still pinning golangci-lint v2.11.4, while the matching longhorn-manager update has already moved to v2.12.2. This PR aligns the engine image build with that newer linter release.

• Linter version bump

  • Updated the root Dockerfile linter image reference from v2.11.4 to v2.12.2.

• Image install flow

  • Replaced the curl-based installer path with a direct copy from the official golangci/golangci-lint image.
  • Keeps the linter supply chain consistent with the upstream change.

FROM golangci/golangci-lint:v2.12.2 AS golangci-lint

COPY --from=golangci-lint /usr/bin/golangci-lint /usr/local/bin/golangci-lint
```<hr>This is an automatic backport of pull request #565 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 0b3ac51 has failed:

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	both modified:   Dockerfile
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