net/make-ca: fix cross-compilation

[mahaase]

I have no idea if this is the correct fix for the problem, that i get the error, that /usr/bin/openssl is not found, if i build the generic sandbox. but in my tests it looks fine I my opinion...

openssl is executed by the package step, so it has to run on whatever we are currently building in. Without the host-compat-toolchain override it would be built for the target sandbox architecture (SANDBOX_ARCH) while the step itself still runs in the host sized bootstrap sandbox. ARCH has to be set explicitly because the host-compat-toolchain does not provide it and openssl derives its Configure target from it.

without the ARCH variable, i get #error "unsupported ARM architecture".