keepass: stop saving config to /nix/store

TLDR: makes KeePass prefer $XDG_CONFIG_HOME instead of install (/nix/store) location

KeePass defaults `PreferUserConfiguration` to `false`,r
 the only way to set it to `true` is through global
 KeePass.config.xml at the install location.
The default installer configuration does just that and nothing else.

Excerpt from documentation (see https://keepass.info/help/base/configuration.html#tech )
> If the PreferUserConfiguration flag is false,
>  try to store all configuration items into the global configuration file.
>  If this fails, report the error and try to store them into the local configuration file.
>  If this fails, report the error.

pkgs/applications/misc/keepass/default.nix

@@ -76,7 +76,11 @@ with builtins; buildDotnetPackage rec {
     ];
   };
 
-  outputFiles = [ "Build/KeePass/Release/*" "Build/KeePassLib/Release/*" ];
+  outputFiles = [
+    "Build/KeePass/Release/*"
+    "Build/KeePassLib/Release/*"
+    "Ext/KeePass.config.xml" # contains <PreferUserConfiguration>true</PreferUserConfiguration>
+  ];
   dllFiles = [ "KeePassLib.dll" ];
   exeFiles = [ "KeePass.exe" ];