Kbuild jobs download optimization

When kbuild start process it need to download tarball artifact, which contain snapshot of kernel source code at specific tree, branch, commit.
This means same cluster building 35 kernel kinds - it will download same tarball at least 35 times.
With average size of tarball 200MB this is 7GB of data transfer (Azure $0.087 per GB = $0.609 per build). Often we have around 40-50 builds per 24h, so this is around $30 per day.
At month this is $900. This is not acceptable, if this is not optimized.
If we slash this at least 30 times, we can save $870 per month.
Potentially it is not happening yet, because we are scheduling build to cluster in same region. But if we start to schedule to different regions, or GCP/AWS we will have this problem.

No files, tests, or entry points are identified. Start by locating the Kbuild artifact-download path and the cluster scheduling or storage configuration; done means repeated builds reuse one tarball without redundant transfers across the stated deployment regions.

Written by the indexing model from the issue text.