scripts: west build: fix pristine builds

There's a couple of missing imports from when the
203021b backport was added to this
branch.

Signed-off-by: Marti Bolivar <[email protected]>

Author: mbolivar-nordic

scripts/west_commands/build.py

@@ -4,6 +4,8 @@
 
 import argparse
 import os
+import shutil
+import subprocess
 
 from west import log
 from west import cmake