### Summary

If a var is defined in the `vars:` of a block, and then redefined again in the `vars:` of an inner block, the outer value is still used.

`vars:` on a _task_ in the inner block work as expected.

### Issue Type

Bug Report

### Component Name

unknown

### Ansible Version

```console
$ ansible --version
ansible [core 2.12.0.dev0] (devel 1fc1ab89ae) last updated 2021/07/17 12:29:56 (GMT -400)
  config file = None
  configured module search path = ['/home/briantist/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/briantist/code/ansible/ansible.core/lib/ansible
  ansible collection location = /home/briantist/.ansible/collections:/usr/share/ansible/collections
  executable location = /home/briantist/code/ansible/ansible.core/bin/ansible
  python version = 3.6.9 (default, Jan 26 2021, 15:33:00) [GCC 8.4.0]
  jinja version = 3.0.1
  libyaml = True
```


### Configuration

```console
$ ansible-config dump --only-changed
CALLBACKS_ENABLED(env: ANSIBLE_CALLBACK_WHITELIST) = ['profile_tasks']
DEFAULT_FORKS(env: ANSIBLE_FORKS) = 30
INVENTORY_ENABLED(env: ANSIBLE_INVENTORY_ENABLED) = ['host_list', 'auto', 'ini', 'yaml', 'toml']
```


### OS / Environment

Ubuntu 18.04 on WSL2

### Steps to Reproduce

<!--- Paste example playbooks or commands between quotes below -->
```yaml
- name: outer
  vars:
    myvar: abc
  block:
    - debug:
        var: myvar

    - name: inner block
      vars:
        myvar: 123
      block:

        - debug:
            var: myvar

        - name: task
          vars:
            myvar: xyz
          debug:
            var: myvar
```

### Expected Results

inner block first `debug` result is `123`

### Actual Results

inner block first `debug` result is `abc`


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
'''
        Creates a new block, with task lists filtered based on the tags.
        '''
        def evaluate_and_append_task(target):
            tmp_list = []
            for task in target:
                if isinstance(task, Block):
                    filtered_block = evaluate_block(task)
                    if filtered_block.has_tasks():
                        tmp_list.append(filtered_block)
                elif ((task.action in C._ACTION_META and task.implicit) or
                        (task.action in C._ACTION_INCLUDE and task.evaluate_tags([], self._play.skip_tags, all_vars=all_vars)) or
                        task.evaluate_tags(self._play.only_tags, self._play.skip_tags, all_vars=all_vars)):
                    tmp_list.append(task)
            return tmp_list
        def evaluate_block(block):
            new_block = block.copy(exclude_parent=True, exclude_tasks=True)
            new_block._parent = block._parent
            new_block.block = evaluate_and_append_task(block.block)
            new_block.rescue = evaluate_and_append_task(block.rescue)
            new_block.always = evaluate_and_append_task(block.always)
            return new_block
        return evaluate_block(self)
    def has_tasks(self):