[Build] Fix a potential issue with too many tags?

This review feedback came out on a PR I made for the nightly builds. It's not related to the changes, but looks like it might need fixing:

> The tag existence check uses gh api /repos/.../tags, which only returns the first page of tags (typically 30). If the repo has more tags, an existing tag may be missed and the pipeline will fail later when creating the release. Use the git ref endpoint to check the specific tag directly (404 => not found). (azd-code-reviewer)

https://github.com/Azure/azure-dev/pull/8740#discussion_r3445247211

Read the nightly-build changes and review discussion in PR #8740 first. Trace the tag existence check and verify the specific git-ref lookup behavior for existing and missing tags; done means pagination no longer causes an existing tag to be treated as absent and the relevant build check passes.

Written by the indexing model from the issue text.