ci: install zstd in the build image

[pitasi]

what

installs zstd in the build image.

why

actions/cache only uses zstd if a zstd binary is on PATH, and it runs inside the job container. the build image never had one, so every job running in it fell back to gzip which is slower and leads to bigger artifacts:

	gzip	zstd
restore step, median	35s	20s
restore step, min/max	31s / 46s	15s / 32s
archive size	1062 MB	1014 MB

that 30s of gunzip is on the critical path for many jobs, and the warmup jobs on main pay it again on the compress side.

notes

• integration jobs are already using zstd so this applies to unit tests
• merging this will invalidate the cache for unit and image-and-lint, but after a first cold start caching will resume as always

[github-actions]

Building new version of mimir-build-image. After image is built and pushed to the registry, a new commit will automatically be added to this PR with new image version us-docker.pkg.dev/grafanalabs-dev/docker-mimir-build-image/mimir-build-image:pr16336-25522b6629. This can take a few minutes.

[github-actions]

Building new version of mimir-build-image. After image is built and pushed to the registry, a new commit will automatically be added to this PR with new image version us-docker.pkg.dev/grafanalabs-dev/docker-mimir-build-image/mimir-build-image:pr16336-400d7ac8a1. This can take a few minutes.

[github-actions]

Not building new version of mimir-build-image. This PR modifies the build image or the build image build workflow, but the image us-docker.pkg.dev/grafanalabs-dev/docker-mimir-build-image/mimir-build-image:pr16336-400d7ac8a1 already exists.

[github-actions]

Building new version of mimir-build-image. After image is built and pushed to the registry, a new commit will automatically be added to this PR with new image version us-docker.pkg.dev/grafanalabs-dev/docker-mimir-build-image/mimir-build-image:pr16336-e30cf26c29. This can take a few minutes.

[github-actions]

Not building new version of mimir-build-image. This PR modifies the build image or the build image build workflow, but the image us-docker.pkg.dev/grafanalabs-dev/docker-mimir-build-image/mimir-build-image:pr16336-e30cf26c29 already exists.