Do not reset framework checkout before running customer tests (#141013)

The migration of customer tests to sharded tests adds a step that checks out the current tip-of-tree of the framework repo, removing local changes. This does not work with monorepo testing, which modifies engine.version, and does not work with local testing of a branch.

The sharded tests should already be running with the correct checkout of the framework repo. If the REVISION environment variable is set, the framework checkout will still be reset to check out that revision.

These commands were migrated from the existing shell script to the sharded tester in
https://github.com/flutter/flutter/pull/138659

Bug: https://github.com/dart-lang/sdk/issues/51042

dev/bots/test.dart

@@ -1577,8 +1577,10 @@ Future<void> _runCustomerTesting() async {
   await runCommand(
     'git',
     <String>[
-      'checkout',
+      'branch',
+      '-f',
       'master',
+      'origin/master',
     ],
     workingDirectory: flutterRoot,
   );