fix: update TDX experimental guide with environment variable declaration

- Added a warning section detailing the syntax for setting environment variables on different operating systems (Mac/Linux and Windows) to enhance user understanding and ensure proper configuration for TDX applications.

Author: akugone

src/build-iapp/guides/using-tdx-experimental.md

@@ -62,6 +62,15 @@ iapp deploy
 iapp run <app-address>
 ```
 
+:::warning Environment Variable Declaration
+
+The syntax for setting environment variables differs between operating systems:
+
+- **Mac/Linux**: `export EXPERIMENTAL_TDX_APP=true`
+- **Windows**: `set EXPERIMENTAL_TDX_APP=true`
+
+:::
+
 ### Per-Command Method
 
 **Enable TDX for specific commands**: