[infra] Fix test matrix path logic in approve_results.

Change-Id: Id84365b760d61e01e06f7c5c1377eae456c15441
Reviewed-on: https://dart-review.googlesource.com/c/83764
Reviewed-by: Alexander Thomas <athom@google.com>
diff --git a/tools/approve_results.dart b/tools/approve_results.dart
index 157d7c5..7220e5c 100755
--- a/tools/approve_results.dart
+++ b/tools/approve_results.dart
@@ -260,9 +260,8 @@
   }
 
   // Load the list of bots according to the test matrix.
-  final testMatrixPath = Platform.script
-      .toFilePath()
-      .replaceAll("approve_results.dart", "bots/test_matrix.json");
+  final testMatrixPath =
+      Platform.script.resolve("bots/test_matrix.json").toFilePath();
   final testMatrix = jsonDecode(await new File(testMatrixPath).readAsString());
   final builderConfigurations = testMatrix["builder_configurations"];
   final testMatrixBots = <String>[];