This is required, since now we're installing `rustfmt` from the latest
state of the fenix repo. This wasn't recent enough for the latest rust
version. The input was locked at (2025-07-02). Now it's up to date.
I verified this by running `rustfmt --version` on my system. Note that I
don't have a system-wide install of rust and only rely on dev shells, so
this can't possibly come from somewhere else.
```
$ rustfmt --version
rustfmt 1.8.0-nightly (6677875279 2025-07-02)
```
This just installs regular rustfmt, which is not needed in this project.
One could say "It doesn't hurt", but in the NixOS dev shell it actually
does since it will shadow nightly rustfmt and we don't have the
`cargo +nightly fmt` synatx on NixOS that is available on other Distros.
Also "It doesn't hurt" to delete it for non NixOS users.
