Use get to access build dictionary (#10797)

ref https://read-the-docs.sentry.io/issues/4473316150/?project=148442&query=is%3Aunresolved&referrer=issue-stream&stream_index=8

readthedocs/projects/tasks/builds.py

@@ -335,7 +335,7 @@ def sigint_received(*args, **kwargs):
             # Only allow to cancel the build if it's not already uploading the files.
             # This is to protect our users to end up with half of the documentation uploaded.
             # TODO: remove this condition once we implement "Atomic Uploads"
-            if self.data.build["state"] == BUILD_STATE_UPLOADING:
+            if self.data.build.get("state") == BUILD_STATE_UPLOADING:
                 log.warning('Ignoring cancelling the build at "Uploading" state.')
                 return