microxrcedds_agent.so fails to link in cross environment

Describe the bug
I tried using a cross compile workflow for Micro ROS Agent and it has linker errors.

To Reproduce
Steps to reproduce the behaviour:

1. Clone repo on this branch and follow the dockerized instructions: https://github.com/Ryanf55/x-ros/tree/micro-ros-agent
2. Compile the micro ros agent as explained
3. Observe the following error:

# [  2%] Linking CXX shared library libmicroxrcedds_agent.so
# /usr/lib/gcc-cross/aarch64-linux-gnu/11/../../../../aarch64-linux-gnu/bin/ld: /opt/ros/humble/lib/libfastrtps.so.2.6.8: error adding symbols: file in wrong format

The linker is incorrectly trying to link to /opt/ros/humble/lib/libfastrtps.so.2.6.8. It should be linking to the shared library in the sysroot, not the host.

Expected behaviour
Successful compilation in a sysroot and linkage only to libraries in the sysroot.

System information (please complete the following information):

• OS: Host OS Ubuntu 22
• ROS 2 humble on host OS
• Target OS: Ubuntu 22

Additional context

I have ROS 2 humble installed on the host OS. MicroROS should compile in the sysroot and ignore all host libraries.
Because it does not ignore host libraries, it is finding libraries of the wrong architecture.

从 Docker 化的交叉编译说明以及生成 libmicroxrcedds_agent.so 的构建命令开始。检查为什么 /opt/ros/humble/lib/libfastrtps.so.2.6.8 出现在链接器输入中，而不是 sysroot 库中，然后为报告的目标架构重现构建过程。只有当 agent 仅使用 sysroot 中的库成功完成链接时，才算完成。