Change build.py ordering for rust and main

Rust now links to C++ instead of C++ to Rust so change the build
ordering to match.

Bug: 183208689
Tag: #floss
Test: atest --host bluetooth_test_gd
Change-Id: I71518b6efc9e45b880baeb2b7db16bbb0006a6e5
diff --git a/build.py b/build.py
index 3e79bff..c6cbabd 100755
--- a/build.py
+++ b/build.py
@@ -414,8 +414,8 @@
         """
         self._target_prepare()
         self._target_tools()
-        self._target_rust()
         self._target_main()
+        self._target_rust()
 
     def build(self):
         """ Builds according to self.target