.github: Work around bst-artifact-server bug in Docker image

bst-artifact-server fails with the default value of `--log-level` due to
an incompatibility with Click 8.0. Work around the issue until the
Docker image is rebuilt with the fix.
diff --git a/.github/compose/ci.buildstream-remote-cache.yml b/.github/compose/ci.buildstream-remote-cache.yml
index 9f853fa..6736ca3 100644
--- a/.github/compose/ci.buildstream-remote-cache.yml
+++ b/.github/compose/ci.buildstream-remote-cache.yml
@@ -19,8 +19,9 @@
 services:
   controller:
     image: buildstream/buildstream:dev
-    command: ["bst-artifact-server","--port",
-      "50052",
+    command: ["bst-artifact-server",
+      "--log-level", "warning",
+      "--port", "50052",
       "--enable-push",
       "/artifacts"
     ]