Rather than pinning a nixpkgs in here, just import the relative path to
the depot to load nixpkgs.
Change-Id: I452727d45e4f44ecc99b86d17e88a413c1911c59
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3025
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI