Fix tag available check to refuse prefix matching

[wagoodman]

Switched the tag-existence check from git/refs/tags/{name} (prefix match,
returns an array) to git/ref/tags/{name} (exact match, returns 404 when
absent). The previous form treated any 200 as "tag exists", producing a
false positive whenever a related pre-release tag was already pushed.

For example, with tags v0.1.0-rc1, v0.1.0-rc2, v0.1.0-rc3 present and v0.1.0 not:

$ gh api "repos/OWNER/REPO/git/refs/tags/v0.1.0" -i | head -1
HTTP/2.0 200 OK    # matches the rc tags by prefix — wrong

$ gh api "repos/OWNER/REPO/git/ref/tags/v0.1.0" -i | head -1
HTTP/2.0 404 Not Found    # correct

Additional fixes:

• ${{ github.repository }} moved into env: as REPO.
• ${HTTP_STATUS:-000} replaces a || echo "000" that was bound to awk
  and never fired on gh api failure.