this derivation will be built:
  /nix/store/cvgbhc4n8zckvwjpx63wlwpa5ijrs4p0-interpolate-conformance.drv
bld4: building cvgbhc4n8zckvwjpx63wlwpa5ijrs4p0-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected