cargo.bbclass: set up cargo environment in common do_compile

cargo.bbclass: set up cargo environment in common do_compile

cargo_do_compile runs only if the recipe is built using cargo
as the top level tool. Some recipes hide usage of cargo inside setuptools
(or autoconf) and use do_compile definitions specific to those,
and so the environment isn't properly set up.

This was exposed by latest versions of python3-cryptography.

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>