tfm: Fix zephyr-sdk toolchain variant integration

The psa-arch-tests project changed how third-party toolchains are
integrated. This broke the Zephyr sdk.

This patch aligns with the new mechanism.

https://github.com/ARM-software/psa-arch-tests/pull/276



Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>