twrp: use ANDROID_ROOT environment variable

instead of hard-coded /system path

I updated most of the references I found,
but there might be more

For devices that have to mount system at /system_root,
this allows system to be bind mounted to /system
and detected properly by TWRP

Change-Id: I9f142fd8cec392f5b88e95476258dab9c21a9aac
diff --git a/twrp.cpp b/twrp.cpp
index 3102efd..2399c0b 100644
--- a/twrp.cpp
+++ b/twrp.cpp
@@ -333,7 +333,7 @@
 #ifndef TW_OEM_BUILD
 	// Check if system has never been changed
-	TWPartition* sys = PartitionManager.Find_Partition_By_Path("/system");
+	TWPartition* sys = PartitionManager.Find_Partition_By_Path(PartitionManager.Get_Android_Root_Path());
 	TWPartition* ven = PartitionManager.Find_Partition_By_Path("/vendor");
 	if (sys) {