This PR updates the Docker Image for NuttX CI, so that it builds Rust Apps correctly for QEMU RISC-V 64-bit. We add the Rust Target for `riscv64gc-unknown-none-elf` to the Docker Image.
In the next PR, we will call the Updated Docker Image to [compile the Rust App `leds_rust`](https://github.com/apache/nuttx/pull/12852), at every run of NuttX CI. This will validate whether Rust Apps are built correctly for QEMU RISC-V 64-bit.
Modified Files:
`tools/ci/docker/linux/Dockerfile`: Add Rust Target `riscv64gc-unknown-none-elf` for Docker CI
`tools/ci/platforms/ubuntu.sh`: Same as above, but for Ubuntu CI
`tools/ci/platforms/msys2.sh`: Same as above, but for MSYS2 CI
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