Support NETRC to download files

[jlegaul7]

As it stands, kvmd does not use netrc to download files, this mr adds the missing flag to let aiohttp use netrc for authentication.

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Reference:
https://docs.aiohttp.org/en/stable/client_reference.html#aiohttp.ClientSession

trust_env (bool) – Trust environment settings for proxy configuration if the parameter is True (False by default).
  Get proxy credentials from ~/.netrc file if present.
  Get HTTP Basic Auth credentials from ~/.netrc file if present.
  If `NETRC` environment variable is set, read from file specified there rather than from ~/.netrc.

[mdevaev]

Maybe settings in override.yaml will be a better solution?