this derivation will be built:
  /nix/store/xs5axd28gqyylmi60rkmmzw1vaknhp7m-interpolate-conformance.drv
bld4: building xs5axd28gqyylmi60rkmmzw1vaknhp7m-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected