remove braces on DRIVERS_TOOLS in prepare resources

Author: Jibola

.evergreen/config.yml

@@ -53,7 +53,7 @@ functions:
         script: |
           . src/.evergreen/scripts/env.sh
           set -o xtrace
-          rm -rf ${DRIVERS_TOOLS}
+          rm -rf $DRIVERS_TOOLS
           if [ "$PROJECT" = "drivers-tools" ]; then
             # If this was a patch build, doing a fresh clone would not actually test the patch
             cp -R ${PROJECT_DIRECTORY}/ ${DRIVERS_TOOLS}