Improve Spec Tooling When Building From Tags

Related to:

> FWIW when a tag is pushed, I wouldn't expect "main" to be the branch, as that has zero information to a user in a PDF. It should either be the tag (which is likely a version) or a commit hash. "main" has very little significance.

_Originally posted by @MathiasMagnus in https://github.com/KhronosGroup/OpenCL-Docs/pull/1351#pullrequestreview-2734828152_

We should improve the spec tooling to better handle the case when building from a tag, which may not have a branch associated with it. This is especially important for automated builds via GitHub actions, which only checks out the tag.

Inspect the spec tooling and automated GitHub Actions build path, then reproduce a build from a checked-out tag rather than a branch. Done means tag builds no longer label the generated PDF as "main" and instead use the tag or a commit hash as described in the issue.

Written by the indexing model from the issue text.