gotify-server: fix UI delivery

After bisecting `nixpkgs-master` I realized that the usage of
`-trimpath`[0] by default for Go modules[1] is responsible for breaking UI
delivery of `gotify-server`[2].

This behavior can only be turned off by setting `allowGoReference` to
`true`.

We may want to find a better long-term fix, but given that this only
affects a leaf-package and actually fixes the problem, this is good
enough for now.

Fixes #105472

[0] https://golang.org/doc/go1.13#go-command
[1] 4e9f7bbf85
[2] https://hydra.nixos.org/build/131660876

pkgs/servers/gotify/default.nix

@@ -20,6 +20,12 @@ buildGoModule rec {
     sha256 = import ./source-sha.nix;
   };
 
+  # With `allowGoReference = true;`, `buildGoModule` adds the `-trimpath`
+  # argument for Go builds which apparently breaks the UI like this:
+  #
+  #   server[780]: stat /var/lib/private/ui/build/index.html: no such file or directory
+  allowGoReference = true;
+
   vendorSha256 = import ./vendor-sha.nix;
 
   doCheck = false;