The format script can be used to automatically format the Nix source
files and also verify that the files are formatted using the `-c`
command argument.
At the moment some files are exempt from the formatting to avoid
causing merge conflicts in active pull requests.
Finally, update the contribution guidelines to note that `nixfmt`
should be used.
This avoids the uncontrollable nature of fetching the tarball as part
of the evaluation. Instead the user can decide when to update and also
perform rollbacks, if necessary.
