make modifiers in s3 artifact manifest a list (#135)

to parse this better in GLCI

Author: yeoldegrove

src/gardenlinux/s3/s3_artifacts.py

@@ -148,7 +148,7 @@ def upload_from_directory(
             "build_timestamp": datetime.fromtimestamp(release_timestamp).isoformat(),
             "gardenlinux_epoch": cname_object.version.split(".", 1)[0],
             "logs": None,
-            "modifiers": feature_set,
+            "modifiers": feature_list,
             "require_uefi": "_usi" in feature_list,
             "secureboot": "_trustedboot" in feature_list,
             "published_image_metadata": None,