feat(infra): Implement parallel base image builds (google#14112)

## What this PR does

This PR refactors the base image build process to support parallel
builds for multiple Ubuntu versions (`legacy`, `20.04`, `24.04`),
aligning with the project's multi-version image strategy.

The primary goal is to enable a flexible and controlled migration path
from the legacy image to newer Ubuntu versions, while ensuring stability
and consistency.

## Why this PR is important

Previously, the base image build was a monolithic process tied to a
single version. This change introduces the necessary infrastructure to:
- Build, test, and publish images for all supported Ubuntu versions
simultaneously.
- Safely promote a new Ubuntu version (e.g., `ubuntu-20-04`) to become
the new default (`:v1` tag) via a single configuration change, without
race conditions.
- Ensure that all versioned tags (e.g., `:ubuntu-24-04`) are also
multi-architecture, providing a consistent experience across all images.

## How the changes were implemented

- **Parallel Build Trigger:** The main `base_builder` Cloud Function in
`infra/build/functions/base_images.py` was refactored. It now iterates
through the `SUPPORTED_VERSIONS` and triggers a separate, parallel
Google Cloud Build run for each version.
- **Configurable Default Version:** A `DEFAULT_VERSION` constant was
introduced. The logic that applies the primary `:v1` tag and builds the
multi-arch manifest for it is now tied to this variable, making the
promotion of a new default version a simple, one-line change.
- **Generalized Manifest Creation:** The logic to create
multi-architecture manifests was generalized and is now applied to *all*
build versions, ensuring that tags like `:ubuntu-20-04` and
`:ubuntu-24-04` are also multi-arch.
- **Simplified Tagging:** The redundant `:legacy` tag was removed. The
build designated as `DEFAULT_VERSION` now produces the `:v1` tag. To
ensure compatibility with chained builds, it also produces the `:latest`
tag. All other builds produce only their specific version tag (e.g.,
`:ubuntu-20-04`).
- **Local Testing Capabilities:** The script now supports `--dry-run`
and `--no-push` flags, allowing for robust local testing and validation
of the build logic without affecting the image registry.
- **Bug Fixes:**
- `report_generator.py` was enhanced to correctly handle build failures
where no projects are run.
- `build_lib.py` was modified to return the full build object, enabling
build tracking and logging of GCB URLs.
- Corrected a path resolution bug for the `base-clang-full` image
variant.

## How to test

1. Run the script locally with the dry run flag to inspect the generated
build steps:
    ```bash
    python3 infra/build/functions/base_images.py --dry-run
    ```
2.  Trigger a real build without publishing the images to the registry:
    ```bash
    python3 infra/build/functions/base_images.py --no-push
    ```
    Monitor the build links logged to the console to confirm success.

## Verification

The new logic was tested by triggering a real build on GCB using the
`--no-push` flag. All three versioned builds completed successfully,
confirming the fix.

- **Legacy (v1) Build:** SUCCESS ([View
Log](https://console.cloud.google.com/cloud-build/builds;region=us-central1/58a597b4-cd61-4177-9941-9bbd22df1df0?project=1053000335369))
- **Ubuntu 20.04 Build:** SUCCESS ([View
Log](https://console.cloud.google.com/cloud-build/builds;region=us-central1/8469368f-a626-4773-8f45-f67a11e03562?project=1053000335369))
- **Ubuntu 24.04 Build:** SUCCESS ([View
Log](https://console.cloud.google.com/cloud-build/builds;region=us-central1/b962938a-dbe0-4347-bc35-7a109cad5b70?project=1053000335369))

## Related Task

- **Taskflow:** [b/441792502](b/441792502)

Author: hunsche