chore(sec): prefix hermetic-build images with infrastructure-public-image (#3732)

This marks the images as infrastructure-related and will not apply for
the customer-facing VM scanning tools. Context in b/395190324 and
[internal
g3doc](https://g3doc.corp.google.com/security/g3doc/isa/autovm/vmprograms/public_image_scanning.md?cl=head#common_vocab)

Author: diegomarquezp

.cloudbuild/library_generation/cloudbuild-library-generation-push.yaml

@@ -20,7 +20,7 @@ substitutions:
   _PRIVATE_LATEST_IMAGE_ID: "${_PRIVATE_IMAGE_NAME}:latest"
   _PRIVATE_VERSIONED_IMAGE_ID: "${_PRIVATE_IMAGE_NAME}:${_GAPIC_GENERATOR_JAVA_VERSION}"
   _PUBLIC_IMAGE_NAME: "gcr.io/cloud-devrel-public-resources/java-library-generation"
-  _PUBLIC_SHA_IMAGE_ID: "${_PUBLIC_IMAGE_NAME}:${COMMIT_SHA}"
+  _PUBLIC_SHA_IMAGE_ID: "${_PUBLIC_IMAGE_NAME}:infrastructure-public-image-${COMMIT_SHA}"
   _PUBLIC_LATEST_IMAGE_ID: "${_PUBLIC_IMAGE_NAME}:latest"
   _PUBLIC_VERSIONED_IMAGE_ID: "${_PUBLIC_IMAGE_NAME}:${_GAPIC_GENERATOR_JAVA_VERSION}"
 steps: