fix(earthly): copy cargo-chef to image layer

A `CACHE` serves as a persistent cache mount which remains popullated
in between layer changes. This is very helpful for speeding up CI when
there are genuine layer updates, since we can, e.g. maintain already
fetched dependencies in `CARGO_HOME`, while adding whichever new ones.

However, there has been a persistent issue with `cargo-chef`, which was
installed into our `CARGO_HOME` shared `CACHE`. Rarely, the `CACHE`
would be cleared by a command like `earthly prune`, but the layer that
installed it was still assumed to be cached (and therefore skipped),
leading to a very confusing error where the installation of `cargo-chef`
was incorrectly skipped, and calls to it would fail.

Copying it immediately after installation outside the persistent `CACHE`
into the layer itself, means that if this step is considered cached by
earthly, and therefore skipped, that a copy of the cargo-chef binary
will be guaranteed to exist in a location already defined in the system
`PATH`, avoiding the confusing and unpredictable error.

Earthfile

@@ -22,7 +22,7 @@ setup:
 
   COPY Cargo.* .rustfmt.toml rust-toolchain.toml .
   RUN rustup show
-  RUN cargo install --locked cargo-chef
+  RUN cargo install --locked cargo-chef && cp "$CARGO_HOME/bin/cargo-chef" /usr/local/bin
 
 source:
   FROM +setup