linters(dart): prefer system dart on PATH, fall back to downloaded SDK

Reorder environment PATH to ["${env.PATH}", "${linter}/bin"] so that a
project-local dart binary (e.g. managed by Flutter) is used when available.
Falls back to trunk's downloaded SDK when no system dart exists. Same
pattern as buf, trivy, and clippy.

Author: AndrewDongminYoo

linters/dart/plugin.yaml

@@ -33,7 +33,7 @@ lint:
         - pubspec.yaml
       environment:
         - name: PATH
-          list: ["${linter}/bin"]
+          list: ["${env.PATH}", "${linter}/bin"]
       commands:
         - name: format
           output: rewrite