PYTHON-5378 Fix usage of drivers secrets (#68)

Author: blink1073

.evergreen/config.yml

@@ -29,11 +29,15 @@ functions:
         directory: "src"
 
   "fetch secrets":
+    - command: ec2.assume_role
+      params:
+        role_arn: ${drivers_test_secrets_role}
     - command: subprocess.exec
       type: setup
       params:
         working_dir: "src"
         binary: bash
+        include_expansions_in_env: [AWS_SECRET_ACCESS_KEY, AWS_ACCESS_KEY_ID, AWS_SESSION_TOKEN]
         args: [.evergreen/fetch-secrets.sh]
 
   "fetch repo":