this derivation will be built:
  /nix/store/4728zyh5mrxfk94iqxdbzmlaajny0wsa-interpolate-conformance.drv
bld3: building 4728zyh5mrxfk94iqxdbzmlaajny0wsa-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected