this derivation will be built:
  /nix/store/4jjr27i7g6i3i82ncc5ch720xwq6qabs-interpolate-conformance.drv
bld4: building 4jjr27i7g6i3i82ncc5ch720xwq6qabs-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected