Defined the key and secret via ENV vars

aws/aws-sdk-php-symfony/1.3/config/packages/aws.yaml

@@ -6,5 +6,5 @@ aws:
     version: latest
     region: us-east-1
     credentials:
-        key: not-a-real-key
-        secret: "@@not-a-real-secret"
+        key: "%env(AWS_CREDENTIAL_KEY)%"
+        secret: "%env(AWS_CREDENTIAL_SECRET)%"

aws/aws-sdk-php-symfony/1.3/manifest.json

@@ -4,5 +4,9 @@
     },
     "copy-from-recipe": {
         "config/": "%CONFIG_DIR%/"
+    },
+    "env": {
+        "AWS_CREDENTIAL_KEY": "not-a-real-key",
+        "AWS_CREDENTIAL_SECRET": "@@not-a-real-secret"
     }
 }