Update Rust from `"nightly-2024-04-15"`.

[meowjesty]

Looks like there is an issue from nightly-2024-07-03 to (at least) nightly-2024-09-03 that is kinda annoying when you try to build the mirrord-agent.

You'll be getting a bunch of errors like this:

item does not constrain `GetDEnts64Stream::{opaque#0}`, but has it in its signature
   --> mirrord/agent/src/file.rs:130:12
    |
130 |     pub fn handle_message(&mut self, request: FileRequest) -> Result<Option<FileResponse>> {
    |            ^^^^^^^^^^^^^^
    |
    = note: consider moving the opaque type's declaration and defining uses into a separate module

Due to some weirdness going on with tracing, e.g. .inspect_err(|fail| error!("file_worker -> {:#?}", fail))?;

It comes from this type alias, more specifically from 1 and 2.

type GetDEnts64Stream = Peekable<
    std::iter::Chain<
        IntoIter<std::result::Result<DirEntryInternal, io::Error>>,
        Map<
            Map<
                Enumerate<ReadDir>,
1 --->          impl Fn((usize, io::Result<DirEntry>)) -> io::Result<DirEntryInternal>,
            >,
2 --->      impl Fn(io::Result<DirEntryInternal>) -> io::Result<DirEntryInternal>,
        >,
    >,
>;