build: Fix name of argument variable

Adding --cmd to the task argument changed the name of the destination
variable, and that broke the whole subcommand because 'cmd' is already
used by the top-level parser. Fix this by setting dest explicitly.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/build.py

@@ -62,7 +62,7 @@ class Build:
         bld_psr.add_argument('--target',
                              action='append',
                              help='Select target to build')
-        bld_psr.add_argument('-c', '--cmd', '--task',
+        bld_psr.add_argument('-c', '--cmd', '--task', dest='task',
                              help='Select which task should be executed')
         bld_psr.add_argument('--skip',
                              help='Skip build steps',