Enable proxy usage for fetch remote resources

[Xeicker]

Description

There was an issue about not being able to run a scan with "Fetch remote resources" enabled, in case the resource need to be get from a proxy. The issue was not present using oscap command, since it was using the usual http_proxy, https_proxy & no_proxy environment variables.

The step in which those variables were lost was in the pkexec command, so with this change, those variables are preserved if they were set