Improve Spec Tooling When Building From Tags

[bashbaug]

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.