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
diff --git a/prebuilt/common/bin/backuptool.sh b/prebuilt/common/bin/backuptool.sh
index c9aa139..1d9dd4e 100755
--- a/prebuilt/common/bin/backuptool.sh
+++ b/prebuilt/common/bin/backuptool.sh
@@ -18,7 +18,7 @@
   mkdir -p /tmp/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