ci: have mergify set the component/build label on PRs

Quite a few PRs have the `build:` prefix. It would be good to teach
Mergify to set the label for those.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

.mergify.yml

@@ -342,6 +342,13 @@ pull_request_rules:
       label:
         add:
           - rebase
+  - name: title contains build
+    conditions:
+      - "title~=build: "
+    actions:
+      label:
+        add:
+          - component/build
   - name: title indicates a bug fix
     conditions:
       - title~=(bug)|(fix)