feat(ops/pipelines): annotate patchset builds with Gerrit URLs

If available, provide a link back to Gerrit on the overview page of a
build.

Uses the default style (i.e. style unset), which makes it
non-intrusive visually.

Change-Id: I4271d589d548015b75762fd0584f3958bfcc53e5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4442
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

ops/pipelines/static-pipeline.yaml

@@ -8,6 +8,12 @@ steps:
   - label: ":llama:"
     command: |
       set -ue
+
+      if test -n "$${GERRIT_CHANGE_URL-}"; then
+        echo "This is a build of [cl/$$GERRIT_CHANGE_ID]($$GERRIT_CHANGE_URL) (at patchset #$$GERRIT_PATCHSET)" | \
+          buildkite-agent annotate
+      fi
+
       nix-build -A ops.pipelines.depot -o pipeline --show-trace
 
       # Steps need to be uploaded in reverse order because pipeline