OECORE_NATIVE_SYSROOT is used by tools like oe-run-native and hence
we were seeing selftest failures when newer buildtools-tarballs that
use this were run on the autobuilder.
Unset the variable after use to avoid these issues.
(From OE-Core rev: ea5128fde10eb9be7fefbbb87ae6181edd2bcdff)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>