builtin:fetchurl should provide more info about where it fetched from

[flokli]

Is your feature request related to a problem? Please describe.
While testing a PR against nixpkgs staging, I was chasing a fixed-output derivation mismatch in the gettext sources: NixOS/nixpkgs#235398

I initially assumed, due to not being "officially released", gettext 0.21.1 changed sources, and proposed a revert to the previous version. After further inspection however, and successfully building on other machines, it looks like this file was simply faulty on one of the gnu mirrors, unluckily one of the ones my Nix has been using.

I extracted the derivation:

❯ nix show-derivation /nix/store/2c90gp52cvdprpj314d86qii9qil4fkb-gettext-0.21.1.tar.gz.drv
{
  "/nix/store/2c90gp52cvdprpj314d86qii9qil4fkb-gettext-0.21.1.tar.gz.drv": {
    "args": [],
    "builder": "builtin:fetchurl",
    "env": {
      "builder": "builtin:fetchurl",
      "executable": "",
      "impureEnvVars": "http_proxy https_proxy ftp_proxy all_proxy no_proxy",
      "name": "gettext-0.21.1.tar.gz",
      "out": "/nix/store/y2h7bqjpc4q9g887w8pbwncjrmr4g9sx-gettext-0.21.1.tar.gz",
      "outputHash": "sha256-6MNlDh2M7odcTzVWQjgsHfgwWL1aEe6FVcDPJ21kbUU=",
      "outputHashAlgo": "sha256",
      "outputHashMode": "flat",
      "preferLocalBuild": "1",
      "system": "builtin",
      "unpack": "",
      "url": "https://ftpmirror.gnu.org/gettext/gettext-0.21.1.tar.gz",
      "urls": "https://ftpmirror.gnu.org/gettext/gettext-0.21.1.tar.gz"
    },
    "inputDrvs": {},
    "inputSrcs": [],
    "outputs": {
      "out": {
        "hash": "e8c3650e1d8cee875c4f355642382c1df83058bd5a11ee8555c0cf276d646d45",
        "hashAlgo": "sha256",
        "path": "/nix/store/y2h7bqjpc4q9g887w8pbwncjrmr4g9sx-gettext-0.21.1.tar.gz"
      }
    },
    "system": "builtin"
  }
}

Unfotunately, neither here, nor in the error message

error: hash mismatch in fixed-output derivation '/nix/store/2c90gp52cvdprpj314d86qii9qil4fkb-gettext-0.21.1.tar.gz.drv':
         specified: sha256-6MNlDh2M7odcTzVWQjgsHfgwWL1aEe6FVcDPJ21kbUU=
            got:    sha256-97ddKLyBBjjVpNWg633BaeMOQwv3nxwH+YUTOpmX3tA=

it's clear where we actually downloaded from (aka, which IP we picked for ftpmirror.gnu.org).

Describe the solution you'd like
The builtin fetcher should probably log the exact endpoint IP it's fetching from.

Describe alternatives you've considered

Additional context

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[flokli]

This could have been nic.funet.fi, as in #8027 (at least the hash mismatch looks similar), but due to the missing info it's hard to tell.