Pass BUILDKITE_BUILD_ID into diff/annotate

This should allow to see, for example, the expected image tag being
built to carry over.

```diff
-    - docker-compose#v4.16.0:
+    - docker-compose#v5.0.0:
         build: base
         config: ".buildkite/docker-compose.yml"
         env:
         - PRE_STEPS
         - RACK
-        image-name: ruby-3-4-build_id
         cache-from:
         - base:973266071021.dkr.ecr.us-east-1.amazonaws.com/builds:ruby-3-4-br-main
         push:
         - base:973266071021.dkr.ecr.us-east-1.amazonaws.com/builds:ruby-3-4-br-
-        image-repository: 973266071021.dkr.ecr.us-east-1.amazonaws.com/builds
```

Notice how the tag is only `ruby-3-4-br-` because the build id was
missing from the environment when generating the pipeline.

Author: zzak

pipelines/buildkite-config/initial.yml

@@ -57,6 +57,7 @@ steps:
           -v "$$PWD":/app:ro -w /app
           -v "$$PWD/tmp":/app/tmp:rw
           -e BUNDLE_FROZEN
+          -e BUILDKITE_BUILD_ID
           ruby:latest
           ./bin/pipeline-annotate
     - command: |
@@ -80,6 +81,7 @@ steps:
           -v "$$PWD/tmp":/app/tmp:rw
           -e RAILS_CI_NIGHTLY
           -e BUNDLE_FROZEN
+          -e BUILDKITE_BUILD_ID
           ruby:latest
           ./bin/pipeline-annotate