GHA: Use GHA environments to select PyPI target

[jorisroovers]

Replace if statements with ${{ vars.PYPI_TARGET }}, defined in
GHA environments, to select the target PyPI repository when publishing
packages.

Relates to #467
Implements #466

[coveralls]

Coverage: 99.662%. Remained the same when pulling 1ab9db7 on ci-environments into 562703f on main.