this derivation will be built:
  /nix/store/ldbpwcgfcnbb9zk3jl40r97vhwmz3kbn-interpolate-conformance.drv
bld4: building ldbpwcgfcnbb9zk3jl40r97vhwmz3kbn-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected