scripts: fix signing in recovery with addon.d-v1

- change to $TMPDIR in addon.d.sh since recovery addon.d-v1 backup + restore leaves you in /tmp/addon.d which the restore then deletes, which would break $BOOTSIGNER execution with the following:
    libc: Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0 in tid 1078 (main), pid 1078 (main)
    Segmentation fault
- also move $BOOTSIGNER execution to after `cd $MAGISKBIN` to ensure it's in a working directory in all cases
- addon.d.sh data mount wasn't doing anything since /data has to already be mounted for the script to be running, so move it into /system/addon.d/99-magisk.sh stub script where it might be useful on recoveries that don't mount /data initially

Fixes #2013

scripts/addon.d.sh

@@ -20,9 +20,8 @@ else
 fi
 
 initialize() {
-  mount /data 2>/dev/null
-
   MAGISKBIN=/data/adb/magisk
+
   if [ ! -d $MAGISKBIN ]; then
     echo "! Cannot find Magisk binaries!"
     exit 1
@@ -46,6 +45,9 @@ main() {
     sleep 5
   fi
 
+  # Ensure we aren't in /tmp/addon.d anymore (since it's been deleted by addon.d)
+  cd $TMPDIR
+
   $BOOTMODE || recovery_actions
 
   ui_print "************************"