test(a3p-integration): update to agoric-upgrade-18a (#11008)

refs: Agoric/agoric-3-proposals#219

## Description

This PR updates a3p to use upgrade-18a image

### Security Considerations

None

### Scaling Considerations

None

### Documentation Considerations

None

### Testing Considerations

Updates a3p image tag for integration tests

### Upgrade Considerations

None

a3p-integration/package.json

@@ -1,7 +1,7 @@
 {
   "private": true,
   "agoricSyntheticChain": {
-    "fromTag": "use-upgrade-18"
+    "fromTag": "use-upgrade-18a"
   },
   "scripts": {
     "build": "yarn run build:sdk && yarn run build:submissions && yarn run build:synthetic-chain",