and LB_LIBRARY_PATH for cpu case in install_fbgemm.sh (#2699)

Summary:
Pull Request resolved: #2699

# context
* `LB_LIBRARY_PATH` is missing the required path for cpu case, leading to test failure at https://github.com/pytorch/torchrec/actions/runs/12914965105
```
ImportError: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /__w/_temp/conda_environment_12914965105/lib/python3.12/site-packages/tensordict/_C.so)
```
* this diff adding the missing branch when there is only cpu available

Reviewed By: vmoens

Differential Revision: D68524715

fbshipit-source-id: dee105434c3beae58fe9c4f1cde07833406f4c76

Author: TroyGarden

Diff for: .github/scripts/install_fbgemm.sh

@@ -15,6 +15,10 @@ if [[ $CU_VERSION = cu* ]]; then
     echo "[NOVA] Setting LD_LIBRARY_PATH ..."
     conda env config vars set -p ${CONDA_ENV}  \
         LD_LIBRARY_PATH="/usr/local/lib:${CUDA_HOME}/lib64:${CONDA_ENV}/lib:${LD_LIBRARY_PATH}"
+else
+    echo "[NOVA] Setting LD_LIBRARY_PATH ..."
+    conda env config vars set -p ${CONDA_ENV}  \
+        LD_LIBRARY_PATH="/usr/local/lib:${CONDA_ENV}/lib:${LD_LIBRARY_PATH}"
 fi
 
 if [ "$CHANNEL" = "nightly" ]; then