fix: use when: platform: for arm64 step routing (Woodpecker 0.15.4 compat)

Top-level platform: key conflicts with commands: in Woodpecker 0.15.4 YAML
parser — treated as a plugin attribute. Move to when: platform: condition
which is supported for agent platform routing in this version.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

.woodpecker/release.yml

@@ -56,11 +56,11 @@ pipeline:
 
   build-linux-arm64:
     image: rust:1.88-slim
-    platform: linux/arm64
     environment:
       TARGET: aarch64-unknown-linux-gnu
     when:
       event: tag
+      platform: linux/arm64
     commands:
       - apt-get update -qq && apt-get install -y -qq libwebkit2gtk-4.1-dev libssl-dev libgtk-3-dev libayatana-appindicator3-dev librsvg2-dev patchelf pkg-config curl perl
       - curl -fsSL https://deb.nodesource.com/setup_22.x | bash -