Build image: Upgrade golangci-lint to v2.6.1

[aknuds1]

What this PR does

Upgrade golangci-lint to v2.6.1 in build image Dockerfile.

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

[github-actions]

Building new version of mimir-build-image. After image is built and pushed to Docker Hub, a new commit will automatically be added to this PR with new image version grafana/mimir-build-image:pr13333-cb707596e8. This can take up to 1 hour.

[github-actions]

Not building new version of mimir-build-image. This PR modifies mimir-build-image/Dockerfile, but the image grafana/mimir-build-image:pr13333-cb707596e8 already exists.

[github-actions]

Not building new version of mimir-build-image. This PR modifies mimir-build-image/Dockerfile, but the image grafana/mimir-build-image:pr13333-cb707596e8 already exists.

[github-actions]

Not building new version of mimir-build-image. This PR modifies mimir-build-image/Dockerfile, but the image grafana/mimir-build-image:pr13333-cb707596e8 already exists.

[github-actions]

Building new version of mimir-build-image. After image is built and pushed to Docker Hub, a new commit will automatically be added to this PR with new image version grafana/mimir-build-image:pr13333-186473b7d4. This can take up to 1 hour.