backuptool: Fix addon.d version check for A-only

* The path /postinstall exists only for A/B, causing:
   grep: /postinstall/tmp/addon.d/*sh: No such file or directory

Change-Id: Ia07b3029e949c3e08302457cd08798a4dde00ef6

prebuilt/common/bin/backuptool.sh

@@ -20,7 +20,7 @@ preserve_addon_d() {
     cp -a $S/addon.d/* /tmp/addon.d/
 
     # Discard any scripts that aren't at least our version level
-    for f in /postinstall/tmp/addon.d/*sh; do
+    for f in /tmp/addon.d/*sh; do
       SCRIPT_VERSION=$(grep "^# ADDOND_VERSION=" $f | cut -d= -f2)
       if [ -z "$SCRIPT_VERSION" ]; then
         SCRIPT_VERSION=1