gotify-server: fix UI

In version 2.0.15 `gotify` switched to `packr` at 2.x which is why the
UI can't be served properly via HTTP and causes an empty 500 response and
the following errors in `journald`:

```
2020/09/12 19:18:33 [Recovery] 2020/09/12 - 19:18:33 panic recovered:
GET / HTTP/1.1
Host: localhost:8080
Accept: */*
User-Agent: curl/7.72.0

stat /home/ma27/Projects/ui/build/index.html: no such file or directory
```

This wasn't caught by the VM-test as it only tested the REST and push
APIs. Using their internal `packr.go` script in our build as it's the
case in the upstream build-system[1] fixes the issue.

[1] https://github.com/gotify/server/pull/277/files#diff-b67911656ef5d18c4ae36cb6741b7965R48

nixos/tests/gotify-server.nix

@@ -41,5 +41,10 @@ import ./make-test-python.nix ({ pkgs, lib, ...} : {
     )
 
     assert title == "Gotify"
+
+    # Ensure that the UI responds with a successfuly code and that the
+    # response is not empty
+    result = machine.succeed("curl -fsS localhost:3000")
+    assert result, "HTTP response from localhost:3000 must not be empty!"
   '';
 })

pkgs/servers/gotify/default.nix

@@ -3,7 +3,6 @@
 , lib
 , fetchFromGitHub
 , buildGoModule
-, packr
 , sqlite
 , callPackage
 }:
@@ -32,12 +31,10 @@ buildGoModule rec {
 
   buildInputs = [ sqlite ];
 
-  nativeBuildInputs = [ packr ];
-
   ui = callPackage ./ui.nix { };
 
   preBuild = ''
-    cp -r ${ui}/libexec/gotify-ui/deps/gotify-ui/build ui/build && packr
+    cp -r ${ui}/libexec/gotify-ui/deps/gotify-ui/build ui/build && go run hack/packr/packr.go
   '';
 
   passthru = {