RUST-1456 Ensure credentials from prior test runs are not used in EC2 AWS auth test (#737)

Author: patrickfreed

.evergreen/config.yml

@@ -204,6 +204,9 @@ functions:
         working_dir: "src"
         script: |
           ${PREPARE_SHELL}
+          # Write an empty prepare_mongodb_aws so no auth environment variables
+          # are set.
+          echo "" > "${PROJECT_DIRECTORY}/prepare_mongodb_aws.sh"
           ASYNC_RUNTIME=${ASYNC_RUNTIME} .evergreen/run-aws-tests.sh
 
   "run aws auth test with aws credentials as environment variables":