Enabling Horde Secrets in UE 5.4

Support for Horde Secrets was broken in UE5.4. To enable it in your codebase,

1. Copy the supplied HordeSetSecretEnvVarTask.cs to Engine/Source/Programs/AutomationTool/BuildGraph/Tasks/HordeSetSecretEnvVarTask.cs
2. Regenerate the BuildGraph schema: Engine\Build\BatchFiles\RunUAT.bat BuildGraph -Schema
3. Check in Engine/Source/Programs/AutomationTool/BuildGraph/Tasks/HordeSetSecretEnvVarTask.cs and Engine/Build/Graph/Schema.xsd