Update test patch script build-image param

This commits updates the build-image call to
use --keep as the build-image was updated
to use --clean by default. As we need to
preserve the ostree_repo during this
build-image call the --keep was added.

Story: 2009969
Task: 45635
Signed-off-by: Luis Sampaio <luis.sampaio@windriver.com>
Change-Id: I102504f3fb933c8b5adbbc68df9dde79e1a34308

sw-patch/scripts/build_test_patches.py

@@ -144,7 +144,7 @@ class TestPatchBuilder():
         """
         cmd = '''
             source import-stx
-            stx shell -c "build-image"
+            stx shell -c "build-image --keep"
         '''
         ret = run_cmd(cmd)
         log.info("Build image return code %s", ret.returncode)