this derivation will be built:
  /nix/store/g2fkhriy7rs77hrsz6j0hc5xmzrzbh7s-interpolate-conformance.drv
bld4: building g2fkhriy7rs77hrsz6j0hc5xmzrzbh7s-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected