clean up blog-only logic in build_base job template

What does this MR do?

• Clean up build_base job anchor logic. Benefits:
  • It's hard to reason about necessary changes and improvements to the build jobs with all of the logic currently going on in nested bash conditionals/etc.
  • Try to clean it up, with the goal of having no multi-line commands or conditionals within the script tag.
    • NOTE: Almost got here, except for the symlink check, which should go away when the symlinks go away.

Related issues

Relates #6689 (closed) #6740 (closed)