.github/compose/ci.bst-artifact-server.yml: Use nightly Docker image

Test the most recent build of bst-artifact-server. This is also
consistent with the Docker image used for BuildGrid service tests.
diff --git a/.github/compose/ci.bst-artifact-server.yml b/.github/compose/ci.bst-artifact-server.yml
index 879e5ca..4eb2468 100644
--- a/.github/compose/ci.bst-artifact-server.yml
+++ b/.github/compose/ci.bst-artifact-server.yml
@@ -18,7 +18,7 @@
 
 services:
   controller:
-    image: buildstream/buildstream:dev
+    image: buildstream/buildstream:nightly
     command: ["bst-artifact-server",
       "--log-level", "warning",
       "--port", "50052",