[posix] set up heap before creating OT instance (#6128)

This is a quick fix that allows heap usage in constructors.

src/posix/platform/system.cpp

@@ -92,13 +92,13 @@ otInstance *otSysInit(otPlatformConfig *aPlatformConfig)
 #endif
     platformRandomInit();
 
-    instance = otInstanceInitSingle();
-    assert(instance != nullptr);
-
 #if OPENTHREAD_CONFIG_HEAP_EXTERNAL_ENABLE
     otHeapSetCAllocFree(calloc, free);
 #endif
 
+    instance = otInstanceInitSingle();
+    assert(instance != nullptr);
+
 #if OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
     platformBackboneInit(instance, aPlatformConfig->mBackboneInterfaceName);
 #endif