imgtool: Install imgtool dependencies in zephyr-build

Install imgtool dependencies before twister tests in zephyr-build to fix
failing ci job due to ImportError, originating from missing new types
added in cryptography library version 40.0.0.

Signed-off-by: Rustam Ismayilov <rustam.ismayilov@arm.com>
Change-Id: I1e4ca88a4a0329a198290f6e1917c139c30abc4f

.github/workflows/zephyr_build.yaml

@@ -91,6 +91,7 @@ jobs:
  export ZEPHYR_TOOLCHAIN_VARIANT=zephyr
  echo "Using Zephyr version: ${{ env.ZEPHYR_VERSION }}"
  echo "Using Mcuboot version: ${{ env.MCUBOOT_VERSION }}"
+ pip install -r ../bootloader/mcuboot/scripts/requirements.txt
  ./scripts/twister --inline-logs -v -N -M --integration --overflow-as-errors --retry-failed 2 ${test_paths}
 
  - name: Upload Tests Results