rust: Fetch cargo from rust-snapshot dir.

rust: Fetch cargo from rust-snapshot dir.

Fixes: Exception: no cargo executable found at
       `${B}/rustc-1.74.1-src/build/x86_64-unknown-linux-gnu/stage0/bin/cargo`

Fix the cargo binary path error on oe-selftest and path set to rust-snapshot dir.

Patch sent to upstream- https://github.com/rust-lang/rust/pull/120125

Signed-off-by: Yash Shinde <Yash.Shinde@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>