How to get stack to use proxy only for `Network` stuff but not for downloading hackages

Question

So I'm inside China and Github is not always accessible here.

I get this when running stack install for a project.

HttpExceptionRequest Request {
  host                 = "raw.githubusercontent.com"
  port                 = 443
  secure               = True
  requestHeaders       = [("User-Agent","The Haskell Stack")]
  path                 = "/fpco/stackage-content/master/stack/stack-setup-2.yaml"
  queryString          = ""
  method               = "GET"
  proxy                = Nothing
  rawBody              = False
  redirectCount        = 10
  responseTimeout      = ResponseTimeoutDefault
  requestVersion       = HTTP/1.1
}
 (InternalException (HostCannotConnect "raw.githubusercontent.com" [Network.Socket.connect: <socket: 15>: does not exist (Connection refused)]))

I'm aware I can setup stack with proxy using http_proxy

$ export http_proxy=IP:PORT
$ stack install

But doing so implies stack would download hackages through the proxy as well, and I happen to be using the tsinghua mirror:

package-indices:
  - download-prefix: http://mirrors.tuna.tsinghua.edu.cn/hackage/
    hackage-security:
        keyids:
        - 0a5c7ea47cd1b15f01f5f51a33adda7e655bc0f0b0615baa8e271f4c3351e21d
        - 1ea9ba32c526d1cc91ab5e5bd364ec5e9e8cb67179a471872f6e26f0ae773d42
        - 280b10153a522681163658cb49f632cde3f38d768b736ddbc901d99a1a772833
        - 2a96b1889dc221c17296fcc2bb34b908ca9734376f0f361660200935916ef201
        - 2c6c3627bd6c982990239487f1abd02e08a02e6cf16edb105a8012d444d870c3
        - 51f0161b906011b52c6613376b1ae937670da69322113a246a09f807c62f6921
        - 772e9f4c7db33d251d5c6e357199c819e569d130857dc225549b40845ff0890d
        - aa315286e6ad281ad61182235533c41e806e5a787e0b6d1e7eef3f09d137d2e9
        - fe331502606802feac15e514d9b9ea83fee8b6ffef71335479a2e68d84adc6b0
        key-threshold: 3 # number of keys required

        # ignore expiration date, see https://github.com/commercialhaskell/stack/pull/4614
        ignore-expiry: no

which means I'm now accessing the Chinese mirror from a proxy in Singapore, and it is really slow.

One quick solution I have is to remove the Chinese mirror and just have everything go through the proxy. But I still want to use the Chinese mirror if it is possible.

So I'm wondering if there is a way to configure stack to use proxy only for Network stuff but not for downloading hackages?