ci: fix GCB substitutions for promotion script (#113)

Author: briandealwis

hack/cloudbuild-promote.yaml

@@ -46,10 +46,12 @@ options:
   #machineType: 'E2_HIGHCPU_8'
 
 substitutions:
-  _STAGING: us-central1-docker.pkg.dev/$PROJECT_ID/skaffold-staging/skaffold-debug-support
-  _PROD: gcr.io/$PROJECT_ID/skaffold-debug-support
+  _STAGING: us-central1-docker.pkg.dev/${PROJECT_ID}/skaffold-staging/skaffold-debug-support
+  _PROD: gcr.io/${PROJECT_ID}/skaffold-debug-support
   _RUNTIMES: go netcore nodejs python
   _IS_LATEST: "1"
+options:
+  dynamic_substitutions: true
 
 steps:
   ###################################################################