fix(gazelle): Consistent substitution pattern for python_default_visibility directive

[dougthor42]

Make the substitution pattern for python_default_visibility consistent
with the existing python_*_naming_convention pattern.

In #1787 I added the python_default_visibility directive and used a
substitution pattern $python_root. However, I missed that the existing
python_*_naming_convention directives include a trailing $.

This PR is just:

rg -l -F "\$python_root" | xargs sed -i 's/\$python_root/$python_root$/g'

[dougthor42]

Notes:

1. We should get this in before 0.32.0 release so that there doesn't have to be any deprecation timeline.
2. I figured this didn't need a changelog entry because it's just part of adding the python_default_visibility directive.