Lines 5-8 and 15 were explained in Section 10.4.5, “Access a non-flake package (not in nixpkgs)”. +
Line 5 adds hello-flake as an input to this flake.
As expected, we need to add hello-flake as a build input, which we do in line 40.
That does make it available at build and runtime, but it doesn’t put it on the path,
so our hello-again script won’t be able to find it.