build-redbear.sh: fix set -e abort when any fork is dirty

The stash loop ended with '[ any_dirty = 0 ] && echo ...' as the
function's last command — when a fork IS dirty (any_dirty=1), the
compound returns 1 and set -e kills the build silently right after the
stash phase. Only visible with REDBEAR_ALLOW_DIRTY=1 (the dirty gate
refuses earlier otherwise). Now an explicit if + return 0.

local/scripts/build-redbear.sh

@@ -175,7 +175,10 @@ redbear_stash_all_dirty_forks() {
             exit 1
         }
     done
-    [ "$any_dirty" = "0" ] && echo "    (all fork sources clean)"
+    if [ "$any_dirty" = "0" ]; then
+        echo "    (all fork sources clean)"
+    fi
+    return 0
 }
 
 redbear_unstash_all_forks() {