cli/builbo: fix default action to help when none specified

ACTION was set to ${DEFAULT_ACTION} ("help") on line 32, then
immediately overwritten with ACTION="" two lines later, causing
the script to have no action when the user provides no action flag.

Remove the overwrite so that ACTION retains its default value of
"help", consistent with DEFAULT_ACTION. This means running builbo
with no action flag now correctly displays the usage/help text.

Author: Copilot

cli/builbo

@@ -32,7 +32,7 @@ function detect_container_command() {
  ACTION="${DEFAULT_ACTION}"
  # action that was explixitly set from the command line:
  SELECTED_ACTION=""
- ACTION="" 
+ ACTION="${DEFAULT_ACTION}"
  # shell for interactive use:
  DEFAULT_I_SHELL="bash"
  I_SHELL="${DEFAULT_I_SHELL}"