The flake has a defaultPackage attribute, so you can nix shell to put the packages it defines on your path.

This can happen if you don't declare a defaultPackage in the flake:

$ nix shell
error: flake 'git+file:///home/dsyer/dev/scratch/flakes-demo' does not provide attribute 'packages.x86_64-linux.default' or 'defaultPackage.x86_64-linux'

You can still use the flake to set up a shell with nix develop:

$ nix develop
$ hello | cowsay
 _______________ 
< Hello, world! >
 --------------- 
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||