[Github] Only run libclang-python-tests on monorepo main

The libclang python binding test CI job currently doesn't have any
restrictions on what branches it will run on when something is pushed
and also isn't restricted to the monorepo. This patch adds a branch
restriction for the push event, only running the CI job when something
is pushed to the main branch (and the path filter is met), and also adds
a filter to ensure that the job comes from a PR against the monorepo or
a push to a branch in the monorepo.