Add Buildkite pipeline for updating hackage.nix and stackage.nix

Author: rvl

.buildkite/updates.yml

@@ -0,0 +1,12 @@
+steps:
+  - label: 'Update hackage.nix'
+    command:
+      - nix-build -A maintainer-scripts.update-hackage -o update-hackage.sh
+      - echo "+++ Updating hackage.nix"
+      - ./update-hackage.sh
+
+  - label: 'Update stackage.nix'
+    command:
+      - nix-build -A maintainer-scripts.update-stackage -o update-stackage.sh
+      - echo "+++ Updating stackage.nix"
+      - ./update-stackage.sh