Extracts the logic for generating our Buildkite pipeline (which has
been copy&pasted and slightly modified in some places outside of
depot) into a generic //nix/buildkite library.
This should cause no change in functionality.
Change-Id: Iad3201713945de41279b39e4f1b847f697c179f7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4726
Autosubmit: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
