feat(ops/pipelines): configure job priority for 🦙

🦙 is the blocking step on which everything else is always waiting, so to
avoid a situation where we have idle builders we should prioritise running
llamas within the available slots to spawn other stuff in the build queue.

Change-Id: I76836275edd0b0aedaf702d2626dacc31ced9fe2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13069
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

ops/pipelines/static-pipeline.yaml

@@ -50,7 +50,8 @@ steps:
   - label: ":llama:"
     key: "pipeline-gen"
     concurrency_group: 'depot-nix-eval'
-    concurrency: 3 # much more than this and whitby will OOM
+    concurrency: 4 # TODO: limit per builder host?
+    priority: 1
     command: |
       set -ue