Add 4 GiB ephemeral storage to verification Lambda

Monthly restore-test downloads and extracts the full dump tarball
in /tmp. As the dump grows with LFS data, the default 512 MB will
eventually cause ENOSPC failures.

lib/constructs/backup-verification.ts

@@ -28,6 +28,7 @@ export class BackupVerification extends Construct {
       handler: "handler",
       index: "app.py",
       memorySize: 512,
+      ephemeralStorageSize: cdk.Size.gibibytes(4),
       timeout: cdk.Duration.minutes(5),
       environment: {
         SOURCE_BUCKET: props.sourceBucket.bucketName,