nexusmods-app: fix passthru tests

[MattSturgeon]

While working on #354015 I noticed that the passthru tests are not actually passing. I don't think this is a new issue, rather I think this has just gone unnoticed until now.

They fail with the font cache dir not being writable, so I set XDG_CACHE_HOME to a writable directory, however they still fail with some fontconfig errors I'm unsure how best to resolve:

$ nix-build -A pkgs.nexusmods-app.passthru.tests
these 4 derivations will be built:
  /nix/store/14l6s73p9qj5cf0iw6a6274jnvksli27-nexusmods-app-test-help.drv
  /nix/store/b72rdl437swq1xa4ijxg67j8igi7m5f4-nexusmods-app-test-associate-nxm.drv
  /nix/store/c33syx9xwmv9rzck846yglnqh9x4ji2a-nexusmods-app-test-serve.drv
  /nix/store/lbjhy26dshv8qhhwcan0zx8yybv4bg9j-nexusmods-app-test-list-tools.drv
building '/nix/store/b72rdl437swq1xa4ijxg67j8igi7m5f4-nexusmods-app-test-associate-nxm.drv'...
building '/nix/store/14l6s73p9qj5cf0iw6a6274jnvksli27-nexusmods-app-test-help.drv'...
building '/nix/store/lbjhy26dshv8qhhwcan0zx8yybv4bg9j-nexusmods-app-test-list-tools.drv'...
building '/nix/store/c33syx9xwmv9rzck846yglnqh9x4ji2a-nexusmods-app-test-serve.drv'...
nexusmods-app-test-associate-nxm> Fontconfig error: Cannot load default config file: No such file: (null)
error: builder for '/nix/store/b72rdl437swq1xa4ijxg67j8igi7m5f4-nexusmods-app-test-associate-nxm.drv' failed with exit code 1;
       last 1 log lines:
       > Fontconfig error: Cannot load default config file: No such file: (null)
       For full logs, run 'nix log /nix/store/b72rdl437swq1xa4ijxg67j8igi7m5f4-nexusmods-app-test-associate-nxm.drv'.
error: build of '/nix/store/14l6s73p9qj5cf0iw6a6274jnvksli27-nexusmods-app-test-help.drv', '/nix/store/b72rdl437swq1xa4ijxg67j8igi7m5f4-nexusmods-app-test-associate-nxm.drv', '/nix/store/c33syx9xwmv9rzck846yglnqh9x4ji2a-nexusmods-app-test-serve.drv', '/nix/store/lbjhy26dshv8qhhwcan0zx8yybv4bg9j-nexusmods-app-test-list-tools.drv' failed

Feedback and ideas would be greatly appreciated!

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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