v0.3.3

This release introduces a functionality where one is able to append the
channel generation to the tag name in the `channel-update` subcommand
and the `switch` subcommand.

If this functionality is applied to a non-switch build
(e.g. `nix-script switch -C -c boot`) this fails.
This is because we fetch the channel generation information from the
running system rather than from the `/nix` filetree.
Clearly an inconvenience! It might be fixed in later releases, though.