cargo.bbclass: use offline mode for building

cargo.bbclass: use offline mode for building

There should not be any network access during the build step so
specify this explicitely to cargo.
This will allow better error message, e.g:

| Caused by:
|  can't checkout from 'ssh://git@.../fmartinsons/zbus-git-dep-test.git': you are in  the offline mode (--offline)

Instead of

|   Updating git repository `ssh://git@.../fmartinsons/zbus-git-dep-test.git`
| warning: spurious network error (2 tries remaining): failed to resolve address for gitlab.com: Temporary failure in name resolution;class=Net (12)

Signed-off-by: Frederic Martinsons <frederic.martinsons@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>