sysupdate: Run ListFeatures in offline mode

sysupdate: Run ListFeatures in offline mode

Historically, the ListFeatures API (in both D-Bus and now varlink) was
run without an `--offline` flag. This appears like it’s an oversight, but
actually `verb_features()` always unconditionally loaded in offline
mode.

In any case, there doesn’t appear to be a reason for the context to be
online (i.e. for it to check sources for available updates). Features are
defined in local config files and are loaded by `read_features()`, which
is called in both offline and online mode.

When the varlink ListFeatures API was added, it was put into online mode
in order to match the lack of `--offline` argument in the existing D-Bus
API implementation. Change both of them to be explicitly in offline mode.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>