Since the Nix command used is an implementation detail of the script,
it's the script that will have to be updated if the command changes,
it's the script that should be opting into this, rather than making
the user do it globally.
Since c0972c16dc ("update-python-libraries: add missing dependency nix"),
Nix will be provided as a dependency, so we know we'll always be using
a modern version and no longer need the fallback.
Avoids situations where a branch and tag share the same
label. Github will silently return a non-valid artifact and
will fail the build.
Since all github releases correspond to an annotated tag, it's
safe to assume that if we found a release for a tag, that the
tag will exist.
All Python packages now have an updateScript. The script calls
`update-python-libraries` and passes it the position of the derivation
expression obtained using `meta.position`. This works fine in case a Nix
expression represents only a single derivation. If there are more in it,
`update-python-libraries` will fail.
