ci: zephyr: update to Zephyr SDK 0.15.0

Updates the CI workflows to use the Zephyr SDK 0.15.0 for
building and testing Zephyr in the CI.

Fixes:
  E: CMake Error at cmake/modules/verify-toolchain.cmake:79 (find_package):
  Could not find a configuration file for package "Zephyr-sdk" that is
  compatible with requested version "0.15".

  The following configuration files were considered but not accepted:

Signed-off-by: Bartosz Bilas <bartosz.bilas@hotmail.com>