Recently added commit a9a387b49 ("Add flake template", 2024-09-12),
when combined with the previous release.nix changes, resulted in
adding the example/flake subdirectory as an output attribute, which
makes no sense.
To do that, we remove the special toplevelOnly parameter introduced in
the last commit and reuse the distro parameter for that. When its
value starts with a dot, it's not a ROS distro name, but an internal
value, which we use to distinguish what to return.
To make "nix flake check" and similar tools work, we need to allow
setting the system attribute in release.nix explicitly. Without this,
flake jobs in Hydra complain about:
error: attribute 'currentSystem' missing' in restricted evaluation mode.
