Update `./generate_workspace.sh` and `./generate_build_files.sh` to reflect the Nix based solution.

[fricklerhandwerk]

Is your feature request related to a problem? Please describe. These seem to be scripts from Google's original codelab. The idea is to output correct solutions to the right files using the scripts. Currently they are not in sync with the builds set up in the actual files, which use rules_nixpkgs.

Describe the solution you'd like Update the scripts to contain the current solution.

Describe alternatives you've considered

• Scripts should not contain the full solution, but the solution with blanks to fill during the exercise. Otherwise we cannot test for correctness in CI.

• Do not use the scripts at all, and instead let learners work off a clean branch.

Additional context Migrated from to-do-list in README.

[rapenne-s]

These scripts doesn't exist anymore after https://github.com/tweag/nix_bazel_codelab/pull/20 and the right build files got fixed in the process.