fix(base-images): Ensure 'latest' tag is pushed for legacy images (#14140)

**Description:**

This PR fixes an issue where the `:latest` tag for `legacy` base images
was being applied during the build but was not being pushed to the
container registry.

The previous logic correctly added the `latest` tag during the image
build step, but this tag was omitted from the final list of images sent
to Google Cloud Build for pushing.

This change updates the `base_builder` function to explicitly add the
`:latest` tagged images to the push list when the build version is
`legacy`, ensuring they are available in the registry as intended.

---

### Verification

The fix was verified by running a manual build and confirming the
outcome.

**1. Successful Cloud Builds:**
* **Legacy Build:**
[b513316c-3375-470e-9fe8-c42d97d5dfc6](https://console.cloud.google.com/cloud-build/builds;region=us-central1/b513316c-3375-470e-9fe8-c42d97d5dfc6?project=1053000335369)
* **Ubuntu 20.04 Build:**
[b2a4ecbb-1e10-4336-b48a-d6f1036f8b77](https://console.cloud.google.com/cloud-build/builds;region=us-central1/b2a4ecbb-1e10-4336-b48a-d6f1036f8b77?project=1053000335369)
* **Ubuntu 24.04 Build:**
[aee65297-221e-44e5-8f7c-747ba294dbec](https://console.cloud.google.com/cloud-build/builds;region=us-central1/aee65297-221e-44e5-8f7c-747ba294dbec?project=1053000335369)

**2. GCR Tag Verification:**
The `gcr.io/oss-fuzz-base/base-builder` image tags were inspected to
confirm they were pushed by the builds above. The timestamps confirm the
successful push of all tags, including `latest` for the legacy build.

```
$ gcloud container images list-tags gcr.io/oss-fuzz-base/base-builder --limit=5 --format="table(digest, tags, timestamp.datetime)"
DIGEST        TAGS                                      DATETIME
354a8acd657e  ubuntu-24-04,ubuntu-24-04-manifest-amd64  2025-10-16 11:33:36+00:00
b35dde0731e1  latest,v1,v1-manifest-amd64               2025-10-16 11:15:22+00:00
0ab0e33cc6a6  ubuntu-20-04,ubuntu-20-04-manifest-amd64  2025-10-16 11:14:36+00:00
```

Author: hunsche