Passing the system's proxy to `pkg2appimage` and change the downloader for downloading the dependencies.

[hongyi-zhao]

I figured out the following BaiduyunNetdisk.yml adapted from here which is also used by this PR:

# BaiduyunNetDisk
# Warning: This application currently cannot run on ubuntu 16.04.7 LTS and lower.ONLY ubuntu 18.04LTS and up are supported. Working hard to fix it.
# More detail:This application require 3.3.0 of the Protocal Buffer runtime libary to run. however ubuntu 16.04.7LTS came with 2.6.1 version. I'm trying to ask the software developer for help. When there's a fix I will patch it.
#Author: linlinger
app: BaiduyunNetdisk

ingredients:
  packages:
    - libgtk-3-0
    - libnotify4
    - libnss3
    - libxss1
    - libxtst6
    - xdg-utils
    - libatspi2.0-0
    - libuuid1
    - libappindicator3-1
    - libsecret-1-0
    
  dist: focal
  sources: 
    - deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
  script:
    - curl -O https://issuepcdn.baidupcs.com/issue/netdisk/LinuxGuanjia/4.3.0/baidunetdisk_4.3.0_amd64.deb
script:
  - cp ./usr/share/applications/baidunetdisk.desktop ./
  - cp ./usr/share/icons/hicolor/scalable/apps/baidunetdisk.svg ./
  -  echo  '#!/bin/bash
  - HERE="$(dirname "$(readlink -f "${0}")")"
  - 
  - export LD_LIBRARY_PATH="$HERE/usr/lib":$LD_LIBRARY_PATH
  - export LD_LIBRARY_PATH="$HERE/usr/lib/i386-linux-gnu":$LD_LIBRARY_PATH
  - export LD_LIBRARY_PATH="$HERE/lib":$LD_LIBRARY_PATH
  - export LD_LIBRARY_PATH="$HERE/lib/i386-linux-gnu":$LD_LIBRARY_PATH
  - "$HERE/opt/baidunetdisk/baidunetdisk" "$@" | cat' > AppRun

Then I try to build the appimage for baidunetdisk with the following steps according to the guidance here

$ curl -O -Lx socks5://127.0.0.1:18888 https://github.com$(curl -Lsx socks5://127.0.0.1:18888 https://github.com/AppImage/pkg2appimage/releases | grep "pkg2appimage-.*-x86_64.AppImage" | head -n 1 | cut -d '"' -f 2)
$ chmod  +x pkg2appimage-*.AppImage
$ ./pkg2appimage-1807-x86_64.AppImage BaiduyunNetdisk.yml

I noticed that pkg2appimage will use wget as the downloader by default. And I want to achieve the following goals: Passing the system's proxy - currently, I've two proxies: http://127.0.0.1:8080 and socks5://127.0.0.1:18888 running on my host - to the download called by pkg2appimage and customize the downloader for downloading the dependencies, say, using curl.

Are there any tips for achieving this goal?

Regards, HZ