Use MBEDTLS_PYTHON_EXECUTABLE

Change one occurrence of ${PYTHON} to ${MBEDTLS_PYTHON_EXECUTABLE}
and add implied ${MBEDTLS_PYTHON_EXECUTABLE} to the start of a
different command.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index c884769..0bd94ba 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -19,6 +19,7 @@
 # Get base names for generated files (starting at "suites/")
 execute_process(
     COMMAND
+        ${MBEDTLS_PYTHON_EXECUTABLE}
         ${CMAKE_CURRENT_SOURCE_DIR}/../tests/scripts/generate_psa_tests.py
         --list-for-cmake
         --directory suites