Inform the user that the environment variable UE_ROOT is not set

[carryaimp]

Fixes: #105

About

Inform the user that the environment variable ue_root is not set.

How Has This Been Tested?

1. The system outputs a message indicating that the UE_ROOT environment variable is not set.

unset UE_ROOT
bash -x ./setup_linux_dev_tools.sh
bash -x ./build.sh clean
bash -x ./build.sh simlibs_debug
# The program will display a message in the standard output .

2. No output.

export UE_ROOT=/home/carryaimp/apps/Linux_Unreal_Engine_5.2.1
bash -x ./setup_linux_dev_tools.sh

bash -x ./build.sh clean
bash -x ./build.sh simlibs_debug

Screenshots and videos (if appropriate):

[jonyMarino]

@carryaimp I think this should be a warning. Because someone may be building the stand-alone sim which is UE independent.

[carryaimp]

@jonyMarino You're right, the code has been adjusted to display a warning message.