Ensure python3 symbolic link exists everywhere

Summary:
On FreeBSD, dependencies can pull python3.9, but not the metapackage
python3. In such case, /usr/bin/env python3 wouldn't be helpful,
as the python3 symlink is created by the metapackage.

Debian, CentOS Stream and Rocky uses also python3 metapackage.

Test Plan: Test on Eglide, db-B-001, docker-002 and dwellers

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2988