mimir-build-image: Explicitly set GOTOOLCHAIN=local

[aknuds1]

What this PR does

In mimir-build-image/Dockerfile, explicitly override environment variable GOTOOLCHAIN to be local, to make sure that the Docker image's bundled Go toolchain is used, rather than the one specified via the toolchain directive in go.mod. The same is already done in official Go images, but being explicit helps also to document.

Which issue(s) this PR fixes or relates to

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• 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:pr9082-9cdfcfb710. This can take up to 1 hour.

[pstibrany]

Thank you very much @aknuds1! This cleans up confusion about which toolchain is used in our builds.

[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:pr9082-9cdfcfb710. This can take up to 1 hour.

[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:pr9082-d0a22a8a96. 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:pr9082-d0a22a8a96 already exists.