fix: use BuildKit instead of legacy builder to build the Hermetic Build images (#3338)

Fixes the following error: ![image](https://github.com/user-attachments/assets/c4d20cbc-e3b3-48d6-9846-896854145857) [Docker Buildkit](https://docs.docker.com/build/buildkit/) is a more advanced version and more actively maintained. We confirmed locally that the error occurs with an old version of the docker builder and that it is fixed when using `DOCKER_BUILDKIT=1`

Author: diegomarquezp

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

@@ -30,6 +30,8 @@ steps:
  "--file", ".cloudbuild/library_generation/library_generation.Dockerfile", "."]
  id: library-generation-build
  waitFor: ["-"]
+ env: 
+ - 'DOCKER_BUILDKIT=1'
 
 options:
  logging: CLOUD_LOGGING_ONLY

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

@@ -30,6 +30,8 @@ steps:
  "--file", ".cloudbuild/library_generation/library_generation.Dockerfile", "."]
  id: library-generation-build
  waitFor: ["-"]
+ env: 
+ - 'DOCKER_BUILDKIT=1'
 
 options:
  logging: CLOUD_LOGGING_ONLY

hermetic_build/DEVELOPMENT.md

@@ -167,9 +167,6 @@ python hermetic_build/library_generation/cli/entry_point.py generate \
  ```
  Please note that the build only works when using the new
  [Docker BuildKit](https://docs.docker.com/build/buildkit/) (enabled through the `DOCKER_BUILDKIT` variable).
- This is meant for local development only (in CloudTops) - GH Actions' Ubuntu-22.04 \
- [comes with the latest Docker version](https://github.com/actions/runner-images/blob/e74605cd6d5407469cf224802f25057bafc23d70/images/ubuntu/Ubuntu2204-Readme.md?plain=1#L81-L83)
- and is able to handle the build properly using the (updated) legacy builder.
 
 3. Set the version of gapic-generator-java
 

hermetic_build/library_generation/tests/integration_tests.py

@@ -202,6 +202,7 @@ def __build_image(cls, docker_file: str, cwd: str):
  subprocess.check_call(
  ["docker", "build", "-f", docker_file, "-t", image_tag, "."],
  cwd=cwd,
+ env=dict(os.environ, DOCKER_BUILDKIT="1"),
  )
 
  @classmethod