Fix ~/.ros_ardusub_env/env breaking on zsh

[brendon-albacore]

The current env file unconditionally does source /opt/ros/jazzy/setup.bash. That setup script uses ${BASH_SOURCE[0]} to find its own location, which is empty in zsh - so AMENT_CURRENT_PREFIX resolves to whatever the shell's cwd is at startup (usually $HOME), and you get a no such file or directory: $HOME/setup.sh error on every new zsh shell. ROS doesn't get sourced via this path; the export lines below still apply, so it sort of half-works and is easy to miss.

The installer already picks the right rc file based on $TARGET_SHELL, but the env body was hardcoded to bash.

Fix

Detect the shell at source time inside env and pick setup.zsh vs setup.bash accordingly:

if [ -n "$ZSH_VERSION" ]; then
    source /opt/ros/jazzy/setup.zsh
else
    source /opt/ros/jazzy/setup.bash
fi

Doing it at source time (rather than at install time via TARGET_SHELL) means the same env works if someone happens to source it from the other shell, and survives a default-shell change without a reinstall.

This could get unwieldy if extended for each shell a user can source. Might recommend removing this source line, and assume user sources in their .*rc files - or doing a check, and confirming it is sourced in the rc files prior to writing to env

Scope

One-line behaviour change in the heredoc that writes env. No reformatting, no other touches.

Tests

No new tests. Verified manually:

• Fresh zsh shell: no error, ros2 available, AMENT_PREFIX_PATH populated.
• Fresh bash shell: unchanged behaviour.