weechat-matrix script cannot find matrix_sso_helper binary

[arachnist]

Describe the bug
weechat-matrix script cannot find the matrix_sso_helper helper binary that's used when logging in to matrix homeservers that require sso

To Reproduce

1. Add an override to weechat to add weechat-matrix weechat script

    (self: super: {
      weechat = super.weechat.override {
        configure = { availablePlugins, ... }: {
          scripts = with super.weechatScripts; [ weechat-otr weechat-matrix multiline ];
        };
      };
    })

2. Try logging in to a homeserver that requires SSO

Actual results
Weechat prints a message:

203055     weechat     │ stderr: Error with command 'matrix_sso_helper'

Expected behavior
weechat-matrix should print a link that user's supposed to click in browser

Notify maintainers
@tilpner @emilazy
@lovek323 @lheckemann

Metadata

• system: "x86_64-linux"
• host os: Linux 5.10.37, NixOS, 21.05pre291486.900115a4f7f (Okapi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.11
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: weechat weechatScripts.weechat-matrix
# a list of nixos modules affected by the problem
module:

[arachnist]

I locally hacked an overlay that works by making the path to matrix_sso_binary static:

    (self: super: {
      weechatScripts.weechat-matrix = super.weechatScripts.weechat-matrix.overrideAttrs (oldAttrs: rec{
        postFixup = oldAttrs.postFixup + ''
          sed -i "s@\"matrix_sso_helper\"@\"$out/bin/matrix_sso_helper\"@" $out/lib/*/site-packages/matrix/server.py
        '';
      });
    })

[symphorien]

This solution looks reasonable (replace the wildcard by the actual file name and use substituteInPlace instead of sed). Would you mind opening a PR fixing this?