Unable to consume officially supported sysext extensions behind proxy

[markafarrell]

Description

I am unable to consume officially supported sysext extensions where flatcar is behind a proxy (i.e. does not have direct internet access)

Impact

This means I am unable to consume the official sysext extensions when my flatcar host does not have direct internet access

Environment and steps to reproduce

Using flatcar version 4054.0.0 on openstack

1. Enable zfs sysext extension by adding it to /etc/flatcar/enabled-sysext.conf using ignition.
2. Configure global proxy settings as per https://www.flatcar.org/docs/latest/setup/systemd/environment-variables/#system-wide-environment-variables
3. Create flatcar server
4. 

Expected behavior

Flatcar is able to download sysext extension image using proxy server

Additional information

I believe the sysext extensions are downloaded by https://github.com/flatcar/bootengine/blob/flatcar-master/dracut/99setup-root/initrd-setup-root-after-ignition#L136