Use --apt-http-proxy/--apt-ftp-proxy for debootstrap

I was surprised to find my --apt-http-proxy config was not actually covering debootstrap downloads. (Though I wouldn't be surprised if I had RTFM, as the docs were technically explicit about this.)

RFC: I could see not wanting scope creep for using the options too broadly, but I think assuming debootstrap falling under apt is reasonable. I would like a second opinion though.