Auto merge of #23327 - servo:daily, r=jdm

Fix Servo Nightly uploads

CC https://github.com/servo/servo/issues/23320.
Closes https://github.com/servo/servo/pull/23326.

Tasks running with this configuration, all green except the last which is still running as of this writing:

* Windows: https://tools.taskcluster.net/groups/dZvq6KJpQVSTzrnKwA5JQg/tasks/RaEwj-wWRSm1qmNytjAG8A/details
* Linux: https://tools.taskcluster.net/groups/JU6A3xS2TTq4dTL3LY39QQ/tasks/YDbSZr9wTHq6U3obG3ZstQ/details
* Android: https://tools.taskcluster.net/groups/FkLah26vSIyKDUwfADUkGw/tasks/JNokEsu2SCmAGL3ZK2Uj7g/details
* macOS: https://tools.taskcluster.net/groups/CtheiEigRYinJjqYlDMuZQ/tasks/XPqUzikPQnybmc6wP8AL-w/details
* WPT update: https://tools.taskcluster.net/groups/JU6A3xS2TTq4dTL3LY39QQ/tasks/MNutIoMWT9KvB19TS2FsXA/details

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python/servo/package_commands.py

@@ -546,7 +546,11 @@ class PackageCommands(CommandBase):
         import boto3
 
         def get_taskcluster_secret(name):
-            url = os.environ["TASKCLUSTER_PROXY_URL"] + "/secrets/v1/secret/project/servo/" + name
+            url = (
+                os.environ.get("TASKCLUSTER_PROXY_URL", "http://taskcluster") +
+                "/secrets/v1/secret/project/servo/" +
+                name
+            )
             return json.load(urllib.urlopen(url))["secret"]
 
         def get_s3_secret():