Only run HTML-specific tasks in builds that output HTML

[AA-Turner]

The no-HTML build failed with an error that switchers.js doesn't exist. In general, it doesn't make sense to run these tasks on the non-HTML builds.

Traceback (most recent call last):
  File "/srv/docsbuild/scripts/build_docs.py", line 644, in run
    self.build()
  File "/srv/docsbuild/scripts/build_docs.py", line 770, in build
    setup_switchers(
  File "/srv/docsbuild/scripts/build_docs.py", line 405, in setup_switchers
    switchers_path.write_text(rendered_template, encoding="UTF-8")
  File "/usr/lib/python3.10/pathlib.py", line 1154, in write_text
    with self.open(mode='w', encoding=encoding, errors=errors, newline=newline) as f:
  File "/usr/lib/python3.10/pathlib.py", line 1119, in open
    return self._accessor.open(self, mode, buffering, encoding, errors,
FileNotFoundError: [Errno 2] No such file or directory: '/srv/docsbuild/cpython-no-html/Doc/build/html/_static/switchers.js'