[NXP] Unset ZEPHYR_TOOLCHAIN_VARIANT using the shell commands

To be able to track shell commands needed to build an example with
`--dry-run`, any environment manipulation shall be done with shell
commands and not using Python specific commands.

scripts/build/builders/nxp.py

@@ -240,11 +240,9 @@ def generate(self):
             if 'ZEPHYR_NXP_BASE' not in os.environ:
                 raise Exception("ZEPHYR_NXP_BASE need to be set")
 
-            if 'ZEPHYR_TOOLCHAIN_VARIANT' in os.environ:
-                del os.environ['ZEPHYR_TOOLCHAIN_VARIANT']
-
             cmd = 'export ZEPHYR_SDK_INSTALL_DIR="$ZEPHYR_NXP_SDK_INSTALL_DIR"'
             cmd += '\nexport ZEPHYR_BASE="$ZEPHYR_NXP_BASE"'
+            cmd += '\nunset ZEPHYR_TOOLCHAIN_VARIANT'
 
             cmd += '\nwest build -p --cmake-only -b {board_name} -d {out_folder} {example_folder}{build_args}'.format(
                 board_name=self.board.Name(self.os_env),