Restore CUDA environment script in documentation (#162)

An important part of the script that sets environment variables was
mistakenly deleted in #141.

This PR restores the missing lines, updated to reference CUDA 13.

---------

Co-authored-by: Rick Ratzel <3039903+rlratzel@users.noreply.github.com>

Author: MathisHammel

docs/cugraph-docs/source/installation/source_build.md

@@ -191,6 +191,9 @@ Next the env_vars.sh file needs to be edited
 vi ./etc/conda/activate.d/env_vars.sh
 
 #!/bin/bash
+# for CUDA 13.x
+export PATH=/usr/local/cuda-13.0/bin:$PATH
+export LD_LIBRARY_PATH=/usr/local/cuda-13.0/lib64:$LD_LIBRARY_PATH
 ```
 
 ```