selftest/rust: correctly form the PATH environment variable

selftest/rust: correctly form the PATH environment variable

There were two issues here:

- the path to native python was not included, so the test
suite was running with host python, causing failures due to
host python being given native libraries:
https://autobuilder.yoctoproject.org/typhoon/#/builders/148/builds/1711/steps/12/logs/stdio
(rust recipe uses native python throughout)

- tmp/hosttools was listead ahead of native-sysroot/usr/bin

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>