FEXServerClient: When running under pressure-vessel don't use FEXServer rootfs

pressure-vessel overrides our rootfs when it does a pivot_root.
Since we are still communicating to the FEXServer we were pulling the
configured rootfs.

Instead check if we are in pressure vessel and avoid doing that.

This fixes FEX running under pressure-vessel.

(There may be some implications to this down the road with code caching
but let's worry about that later)

External/FEXCore/Source/Interface/Config/Config.cpp

@@ -371,6 +371,22 @@ namespace JSON {
     return {};
   }
 
+
+  std::string FindContainer() {
+    // We only support pressure-vessel at the moment
+    const static std::string ContainerManager = "/run/host/container-manager";
+    if (std::filesystem::exists(ContainerManager)) {
+      std::vector<char> Manager{};
+      if (FEXCore::FileLoading::LoadFile(Manager, ContainerManager)) {
+        // Trim the whitespace, may contain a newline
+        std::string ManagerStr = Manager.data();
+        ManagerStr = FEXCore::StringUtils::Trim(ManagerStr);
+        return ManagerStr;
+      }
+    }
+    return {};
+  }
+
   std::string FindContainerPrefix() {
     // We only support pressure-vessel at the moment
     const static std::string ContainerManager = "/run/host/container-manager";