Tweak to hacking path fix to go back to subshell use

hacking/env-setup

@@ -25,7 +25,7 @@ fi
 # The below is an alternative to readlink -fn which doesn't exist on OS X
 # Source: http://stackoverflow.com/a/1678636
 FULL_PATH=$(python -c "import os; print(os.path.realpath('$HACKING_DIR'))")
-export ANSIBLE_HOME="`dirname "$FULL_PATH"`"
+export ANSIBLE_HOME="$(dirname "$FULL_PATH")"
 
 PREFIX_PYTHONPATH="$ANSIBLE_HOME/lib"
 PREFIX_PATH="$ANSIBLE_HOME/bin"