[artifact tracker] Add back artifact bucket config (#1601)

Summary: The `PL_ARTIFACT_BUCKET` config was mistakenly removed in a
previous change. This causes issues for users deploying self-hosted
cloud.

Type of change: /kind cleanup

Test Plan: Followed the self-hosted cloud install guide successfully.

Signed-off-by: James Bartlett <jamesbartlett@pixielabs.ai>

Author: JamesMBartlett

k8s/cloud/base/artifact_config.yaml

@@ -5,4 +5,5 @@ metadata:
   name: pl-artifact-config
 data:
   PL_ARTIFACT_MANIFEST_URL: https://artifacts.px.dev/artifacts/manifest.json
+  PL_ARTIFACT_BUCKET: pixie-dev-public
   PL_SA_KEY_PATH: /creds/service_account.json