zathura-pdf-poppler: fix source url

[rvolosatovs]

Motivation for this change

Current zathura-pdf-poppler URL constantly times out - changed it to use their Gitlab directly.
Not sure why hash changed

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[JohnAZoidberg]

I can confirm that the latency for the regular site is pretty bad and git is quite good.
It still builds fine - haven't tested it though.

The hash changed because it's a different compression format xz -> gz.