nix: Pin cargo-about to 0.8.2 (zed-industries#44901)

`cargo-about` got pinned to 0.8.2 in
zed-industries#44012, but this isn't exactly
"easy" to accomplish in nix. The version of nixpkgs in the flake inputs
uses the proper version, but if you override the nixpkgs input or use
the provided overlay, you might end up trying to build with a bad
version of `cargo-about`.

Since nixpkgs is versioned as a whole, your options are (in rough order
of desirability):
1. Hope that nixpkgs simply includes multiple versions of the same
package (common for things with stable major versions/breaking changes)
1. Use either `override` or `overrideAttrs` to provide different
version/source attributes
1. Depend on multiple versions of nixpkgs to get the specific versions
of the packages you want
1. Vendor the whole package build from a specific point in its history

Option 1 is out - there's only one version of cargo-about in nixpkgs.

Option 2 doesn't seem to work due to the way that `buildRustPackage`
wraps the base `mkDerivation` which provides the `override` extension
functions. There *might* be a way to make this work, but I haven't dug
into the `buildRustPackage` internals enough to say for sure. Edit: I
apparently can't read and the problems with this option were already
solved for `cargo-bundle`, so this is the final approach!

Option 3 always just feels a bit icky and opaque to me.

Leaving Option 4. I usually find this approach to be "fine" for small
package definitions that aren't actually much bigger than the overridden
attributes would have be with the Option 2 approach. ~~Since the
`cargo-about` definition is nice and small, this is the approach I
chose.~~

~~Since this has the potential to require a build of `cargo-about`, I'm
only actually invoking its build if the provided version is wrong - more
or less the same thing that's happening in the `generate-licenses`
script, but nix-y.~~
Edit: Shouldn't ever cause a rebuild since there's only one 0.8.2 input
source/vendored deps, so anything that was already using it will already
be cached.

I'm also updating nixpkgs to the latest unstable which currently has
`cargo-about 0.8.4` to prove that this works.

Unrelatedly, I also ran `nix fmt` as a drive-by change. `nix/build.nix`
was a bit out of spec.

Release Notes:

- N/A

Author: jrobsonchase