Tools: install-preresq-ubuntu: add check for DO_AP_STM_ENV before adding to path

This prevent to have /opt//bin on PATH if we don't want STM toolchain

Tools/environment_install/install-prereqs-ubuntu.sh

@@ -256,6 +256,8 @@ heading "Adding ArduPilot Tools to environment"
 
 SCRIPT_DIR=$(dirname $(realpath ${BASH_SOURCE[0]}))
 ARDUPILOT_ROOT=$(realpath "$SCRIPT_DIR/../../")
+
+if [[ $DO_AP_STM_ENV -eq 1 ]]; then
 exportline="export PATH=$OPT/$ARM_ROOT/bin:\$PATH";
 grep -Fxq "$exportline" ~/$SHELL_LOGIN 2>/dev/null || {
     if maybe_prompt_user "Add $OPT/$ARM_ROOT/bin to your PATH [N/y]?" ; then
@@ -265,6 +267,7 @@ grep -Fxq "$exportline" ~/$SHELL_LOGIN 2>/dev/null || {
         echo "Skipping adding $OPT/$ARM_ROOT/bin to PATH."
     fi
 }
+fi
 
 exportline2="export PATH=$ARDUPILOT_ROOT/$ARDUPILOT_TOOLS:\$PATH";
 grep -Fxq "$exportline2" ~/$SHELL_LOGIN 2>/dev/null || {