Adds a Nix cache proxy which can be used to send a Nix cache lookup to the first
available cache that has the given NAR. We will use this for dynamically created
builders.
Relates to b/432.
Change-Id: If970d2393e43ba032b5b7d653f2b92f6ac0eab63
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12949
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>
