Nixery is going to gain a new binary (used for building images without
a registry server); to prepare for this the server binary has moved to
cmd/server and the Nix build logic has been updated to wrap this
binary and set the required environment variables.
Change-Id: I9b4f49f47872ae76430463e2fcb8f68114070f72
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5603
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
