scripts: build_env.sh: clamp empy to 3.3.4

em.ParseError: unknown markup sequence: `@)`; the `@)` markup has been removed, just use `)` instead

Author: peterbarker

scripts/install_build_env.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-python3 -m pip install empy
+python3 -m pip install empy==3.3.4
 python3 -m pip install pymavlink
 python3 -m pip install dronecan
 python3 -m pip install pyserial