fix(build): rerun rclrs's build script when AMENT_PREFIX_PATH changes

[rosterloh]

Problem

rclrs/build.rs emits a rustc-link-search path for every prefix
get_search_paths() returns from AMENT_PREFIX_PATH, but declares only ROS_DISTRO as a rerun
trigger. Once a build script emits any rerun-if line, Cargo reruns it
only for the listed triggers. So the search paths stay whatever they were
on the first build.

ros-env does declare rerun-if-env-changed=AMENT_PREFIX_PATH. So after
sourcing a workspace that adds an interface package, ros-env reruns and
compiles that package in, with a #[link] to its typesupport library, but no
build script offers the new prefix's lib/ to the linker:

ld: library not found for -leit_capture_interfaces__rosidl_typesupport_c

The library is on the new prefix. cargo clean -p rclrs -p rosidl_runtime_rs
fixes it, which is how it has been worked around.

Reproduction

A crate depending on rclrs 0.8.0 and ros-env 0.3.0, in a fresh target
directory:

1. Build with only the base ROS environment sourced → ok.
2. Source a workspace that adds an interface package, rebuild without cleaning.

Build	step 2
crates.io rclrs 0.8.0 + rosidl_runtime_rs 0.7.0	fails (library not found)
the rosidl_runtime_rs companion only	links
this PR only	links
both	links

Cargo collects link search paths from every build script, so this repro links
once either crate reruns. The rosidl_runtime_rs companion is the one that
clears the symptom for everyone: every rclrs user also builds it, and crates
that use ros-env without rclrs get their search paths only from it.

This PR is still needed on its own terms. rclrs/build.rs reads
AMENT_PREFIX_PATH and should declare it as a rerun trigger. Without that,
rclrs keeps handing the linker the old prefixes, and an older copy of a
library left in one of them can be picked ahead of the new one.

Change

One println!("cargo:rerun-if-env-changed={AMENT_PREFIX_PATH_ENV_VAR}"), using
ament_rs's own constant, next to the get_search_paths() call that reads it.

Testing

• The reproduction above (ROS 2 Lyrical, RoboStack, macOS arm64).
• cargo test -F default (RoboStack Lyrical with test_msgs and
  example_interfaces, Rust 1.98.1 stable, macOS arm64): 120 tests and 51
  doc-tests pass, the same as on main.
• cargo clippy --no-deps --all-targets -F default -- -D warnings fails with
  the same 123 findings on main and on this branch under clippy 0.1.98
  (the library's deref and doc-list lints). None of them are in build.rs,
  so main doesn't pass this gate on current stable either.
• Stable cargo fmt --check: clean. CI uses nightly rustfmt, which I didn't run.