Add support to ftp_proxy env variable

[Marco Trevisan (Treviño)]

The builders properly set the http_proxy and https_proxy env variables, but not the ftp_proxy one.

By manually setting this to the same value of http_proxy makes ftp transfers properly working (and building snaps from ftp repos) as expected.

[Colin Watson (cjwatson)]

I'm not sure how this would have worked as-is, since it looks like we'd also need to update the proxy configuration to allow port 21. But that seems like a thing we can do.