ci: require the GitHub Actions PR build for mergify (#34836)

### Reason for this change

Tell mergify about our new GitHub actions based build.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mrgrain

.mergify.yml

@@ -16,6 +16,7 @@ queue_rules:
       - "#changes-requested-reviews-by=0"
       - status-success~=AWS CodeBuild us-east-1
       - status-success=validate-pr
+      - status-success=build
     commit_message_template: |-
       {{ title }} (#{{ number }})
       {{ body }}
@@ -36,6 +37,7 @@ queue_rules:
       - "#changes-requested-reviews-by=0"
       - status-success~=AWS CodeBuild us-east-1
       - status-success=validate-pr
+      - status-success=build
     commit_message_template: |-
       {{ title }} (#{{ number }})
       {{ body }}
@@ -55,6 +57,7 @@ queue_rules:
       - "#changes-requested-reviews-by=0"
       - status-success~=AWS CodeBuild us-east-1
       - status-success=validate-pr
+      - status-success=build
     commit_message_template: |-
       {{ title }} (#{{ number }})
       {{ body }}
@@ -87,6 +90,7 @@ pull_request_rules:
       - "#changes-requested-reviews-by=0"
       - status-success~=AWS CodeBuild us-east-1
       - status-success=validate-pr
+      - status-success=build
   - name: automatic priority merge
     actions:
       comment:
@@ -108,6 +112,7 @@ pull_request_rules:
       - "#changes-requested-reviews-by=0"
       - status-success~=AWS CodeBuild us-east-1
       - status-success=validate-pr
+      - status-success=build
   - name: automatic merge (2+ approvers)
     actions:
       comment:
@@ -129,6 +134,7 @@ pull_request_rules:
       - "#changes-requested-reviews-by=0"
       - status-success~=AWS CodeBuild us-east-1
       - status-success=validate-pr
+      - status-success=build
   - name: automatic merge (no-squash)
     actions:
       comment:
@@ -150,6 +156,7 @@ pull_request_rules:
       - "#changes-requested-reviews-by=0"
       - status-success~=AWS CodeBuild us-east-1
       - status-success=validate-pr
+      - status-success=build
   - name: remove stale reviews
     actions:
       dismiss_reviews:
@@ -190,6 +197,7 @@ pull_request_rules:
       - "#changes-requested-reviews-by=0"
       - status-success~=AWS CodeBuild us-east-1
       - status-success=validate-pr
+      - status-success=build
 priority_rules:
   - name: priority for queue `default-merge`
     conditions: