fix: set ros_distro RUSTFLAGS in release-plz like rclrs (#29)

cargo publish --features use_ros_shim has no ROS_DISTRO, and docs.rs
rustc-args are not applied then. Match rclrs by passing
--cfg ros_distro="humble" through the release workflow.

Assisted-by: Cursor:Grok-4.6 [Cursor Agent]

Author: azerupi

.github/workflows/release-plz.yml

@@ -5,6 +5,9 @@ on:
     branches:
       - main
 
+env:
+  RUSTFLAGS: "--cfg ros_distro=\"humble\""
+
 jobs:
   release-plz-release:
     name: Release-plz release