Do not export any variables in automation scripts.

The `export` in the previous steps can overwrite default make vales in the next steps in [the scripts](https://github.com/3mdeb/CROSSCON-Hypervisor-and-TEE-Isolation-Demos/tree/master/env). Example: [link](https://github.com/3mdeb/CROSSCON-Hypervisor-and-TEE-Isolation-Demos/blob/54eb0b2af44066d2c2e671fea691e3d58e9fc629/env/build_rpi4.sh#L391).