chore(coverage): use getter for _dartExecutable to respect cli_util Zone overrides

pkgs/coverage/bin/test_with_coverage.dart

diff --git a/pkgs/coverage/bin/test_with_coverage.dart b/pkgs/coverage/bin/test_with_coverage.dart
index 937598a..63deeec 100644
--- a/pkgs/coverage/bin/test_with_coverage.dart
+++ b/pkgs/coverage/bin/test_with_coverage.dart
@@ -16,7 +16,7 @@
 import 'format_coverage.dart' as format_coverage;
 
 final _allProcesses = <Process>[];
-final _dartExecutable = dartExecutable ?? 'dart';
+String get _dartExecutable => dartExecutable ?? 'dart';
 
 Future<void> _dartRun(
   List<String> args, {

pkgs/coverage/lib/src/run_and_collect.dart

diff --git a/pkgs/coverage/lib/src/run_and_collect.dart b/pkgs/coverage/lib/src/run_and_collect.dart
index 4afad6c..83afdce 100644
--- a/pkgs/coverage/lib/src/run_and_collect.dart
+++ b/pkgs/coverage/lib/src/run_and_collect.dart
@@ -10,7 +10,7 @@
 import 'collect.dart';
 import 'util.dart';
 
-final _dartExecutable = dartExecutable ?? 'dart';
+String get _dartExecutable => dartExecutable ?? 'dart';
 
 Future<Map<String, dynamic>> runAndCollect(
   String scriptPath, {