toolchain-script: use immediate expansion for TOOLCHAIN_CONFIGSITE_NOCACHE

toolchain-script: use immediate expansion for TOOLCHAIN_CONFIGSITE_NOCACHE

Use immediate expansion for the assignment to avoid other classes modifying
variables like HOST_ARCH which changes the result. This happens in the
meta-environment recipe. It inherits cross-canadian which changes HOST_ARCH
from TARGET_ARCH to SDK_ARCH, resulting in wrong site config files in the
generated SDK.

[YOCTO #10255]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>