chore(build): nuget.config 内网 Nexus HTTP 源补 allowInsecureConnections，修复 NU1302

NuGet 新版（VS 设计时 restore / SDK 7+）强制 HTTPS 源，内网
git.yiliantech.com/nexus 仅有 HTTP，导致 YLErp.Resources/MailKit/Security/Cache/Core
还原报 NU1302。按官方指定开关显式放行该源；nuget.org 仍走 HTTPS 不受影响。
验证：dotnet restore --force 真实触源成功，全解决方案编译 0 错误。

nuget.config

@@ -3,6 +3,6 @@
   <packageSources>
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
-    <add key="yilian-nexus" value="http://git.yiliantech.com/nexus/repository/nuget-group/index.json" protocolVersion="3" />
+    <add key="yilian-nexus" value="http://git.yiliantech.com/nexus/repository/nuget-group/index.json" protocolVersion="3" allowInsecureConnections="true" />
   </packageSources>
 </configuration>