[web] Cleaner output on LUCI (#41989)

Using ansi colors in LUCI results in output that looks like:
```
00:00 �[32m+0�[0m: �[1m�[90mloading flutter_tester_emulation_golden_test.dart�[0m�[0m                                                                                                                                            
00:01 �[32m+0�[0m: �[1m�[90mloading flutter_tester_emulation_golden_test.dart�[0m�[0m                                                                                                                                            
00:02 �[32m+0�[0m: �[1m�[90mloading flutter_tester_emulation_golden_test.dart�[0m�[0m
```

This PR disables colors on LUCI so that we get clean output that's easily scannable and searchable.

lib/web_ui/dev/steps/run_suite_step.dart

@@ -84,6 +84,7 @@ class RunSuiteStep implements PipelineStep {
       '--platform=${browserEnvironment.packageTestRuntime.identifier}',
       '--precompiled=$bundleBuildPath',
       '--configuration=$configurationFilePath',
+      if (AnsiColors.shouldEscape) '--color' else '--no-color',
 
       // TODO(jacksongardner): Set the default timeout to five minutes when
       // https://github.com/dart-lang/test/issues/2006 is fixed.

lib/web_ui/dev/utils.dart

@@ -10,6 +10,7 @@ import 'package:args/command_runner.dart';
 import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
 
+import 'common.dart';
 import 'environment.dart';
 import 'exceptions.dart';
 import 'felt_config.dart';
@@ -428,7 +429,13 @@ io.Directory getSkiaGoldDirectoryForSuite(TestSuite suite) {
 }
 
 extension AnsiColors on String {
-  static bool shouldEscape = io.stdout.hasTerminal && io.stdout.supportsAnsiEscapes;
+  static bool shouldEscape = () {
+    if (isLuci) {
+      // Produce clean output on LUCI.
+      return false;
+    }
+    return io.stdout.hasTerminal && io.stdout.supportsAnsiEscapes;
+  }();
 
   static const String _noColorCode = '\u001b[39m';