rust: use rust-snapshot binaries only in rust-native

rust: use rust-snapshot binaries only in rust-native

Otherwise, use rust-native and cargo-native binaries as that allows
our native tweaks in them to be used for target/nativesdk rust -
same as for everything else written in rust.

In particular, this allows building target rust with
cargo-native that includes important reproducibility tweaks.

Unfortunately, this also breaks rust selftest, and that
is partially addressed by the following commit.

[YOCTO #15185]

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>