tools/ci: Add Rust Target for QEMU RISC-V 64-bit

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

tools/ci/docker/linux/Dockerfile

@@ -66,7 +66,8 @@ ENV RUSTUP_HOME=$RUST_HOME/rustup
 RUN mkdir -p $CARGO_HOME \
   && curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y \
   && $CARGO_HOME/bin/rustup target add thumbv6m-none-eabi \
-  && $CARGO_HOME/bin/rustup target add thumbv7m-none-eabi
+  && $CARGO_HOME/bin/rustup target add thumbv7m-none-eabi \
+  && $CARGO_HOME/bin/rustup target add riscv64gc-unknown-none-elf
 
 RUN mkdir /tools/gn -p \
   && cd /tools/gn \

tools/ci/platforms/msys2.sh

@@ -181,6 +181,7 @@ rust() {
     # Install targets supported from NuttX
     "$CARGO_HOME"/bin/rustup target add thumbv6m-none-eabi
     "$CARGO_HOME"/bin/rustup target add thumbv7m-none-eabi
+    "$CARGO_HOME"/bin/rustup target add riscv64gc-unknown-none-elf
     rm rustup-init.exe
   fi
   command rustc --version

tools/ci/platforms/ubuntu.sh

@@ -230,6 +230,7 @@ rust() {
     # Install targets supported from NuttX
     rustup target add thumbv6m-none-eabi
     rustup target add thumbv7m-none-eabi
+    rustup target add riscv64gc-unknown-none-elf
   fi
 
   command rustc --version