Emit a .dsv env hook so the plugin path resolves under colcon

[MonkeScripts]

Problem

The environment hook was a .sh-only file that hardcoded the configure-time install prefix:

export GZ_SIM_SYSTEM_PLUGIN_PATH="@CMAKE_INSTALL_PREFIX@/lib:${GZ_SIM_SYSTEM_PLUGIN_PATH}"

That works, but bakes an absolute path into the installed hook, so the install tree cannot be moved, copied into an image, or reused under a different mount without rebuilding.

Fix

Switch to the ament-native hook and declare the DSV descriptor so ament_environment_hooks() also emits gz_led_plugin.dsv:

set(AMENT_CMAKE_ENVIRONMENT_HOOKS_DESC_gz_led_plugin
  "prepend-non-duplicate;GZ_SIM_SYSTEM_PLUGIN_PATH;lib")

This is the same mechanism ament uses for its own ament_prefix_path / path / library_path hooks, which all ship both forms.

Why the descriptor matters

_local_setup_util_sh.py prefers a sibling .dsv and only falls back to sourcing the .sh:

if os.path.exists(basename + '.dsv'):
    extensions.add('dsv')
...
if 'dsv' in extensions:
    commands += process_dsv_file(basename + '.dsv', ...)
elif ...:
    commands += [FORMAT_STR_INVOKE_SCRIPT.format_map(...)]

and that fallback exports COLCON_CURRENT_PREFIX, never AMENT_CURRENT_PREFIX:

FORMAT_STR_INVOKE_SCRIPT = 'COLCON_CURRENT_PREFIX="{prefix}" _colcon_prefix_sh_source_script "{script_path}"'

So a .sh-only hook using ament_prepend_unique_value "$AMENT_CURRENT_PREFIX/lib" would silently prepend whichever prefix was sourced last (e.g. /opt/ros/humble/lib). Emitting the .dsv avoids that path entirely.

The descriptor value is relative (lib), which colcon resolves against the package prefix — so the install tree stays relocatable.

Verification

Clean rebuild, then loading a world that uses the plugin (headless, isolated GZ_PARTITION):

• gz_led_plugin.dsv emitted alongside the .sh
• GZ_SIM_SYSTEM_PLUGIN_PATH head is install/gz_led_plugin/lib; no /opt/ros/humble/lib contamination
• libgz_led_plugin.so resolvable on the path
• 0 Failed to load system plugin errors, 0 LedPlugin errors