Run gsutil with full python3 and full path. (#132805)

This is to prevent failures when the gsutil.py is not executable.

dev/bots/prepare_package.dart

@@ -833,7 +833,6 @@ class ArchivePublisher {
       print('gsutil.py -- $args');
       return '';
     }
-    if (platform.isWindows) {
     return _processRunner.runProcess(
       <String>['python3', path.join(platform.environment['DEPOT_TOOLS']!, 'gsutil.py'), '--', ...args],
       workingDirectory: workingDirectory,
@@ -841,13 +840,6 @@ class ArchivePublisher {
     );
   }
 
-    return _processRunner.runProcess(
-      <String>['gsutil.py', '--', ...args],
-      workingDirectory: workingDirectory,
-      failOk: failOk,
-    );
-  }
-
   /// Determine if a file exists at a given [cloudPath].
   Future<bool> _cloudPathExists(String cloudPath) async {
     try {

dev/bots/test/prepare_package_test.dart

@@ -38,7 +38,7 @@ void main() {
     final FakePlatform platform = FakePlatform(
       operatingSystem: platformName,
       environment: <String, String>{
-        'DEPOT_TOOLS': path.join('D:', 'depot_tools'),
+        'DEPOT_TOOLS': platformName == Platform.windows ? path.join('D:', 'depot_tools'): '/depot_tools',
       },
     );
     group('ProcessRunner for $platform', () {
@@ -378,7 +378,7 @@ void main() {
       late Directory tempDir;
       final String gsutilCall = platform.isWindows
           ? 'python3 ${path.join("D:", "depot_tools", "gsutil.py")}'
-          : 'gsutil.py';
+          : 'python3 ${path.join("/", "depot_tools", "gsutil.py")}';
       final String releasesName = 'releases_$platformName.json';
       final String archiveName = platform.isLinux ? 'archive.tar.xz' : 'archive.zip';
       final String archiveMime = platform.isLinux ? 'application/x-gtar' : 'application/zip';