Fixes issue with multiple alerts

[ludeeus]

Description:

When the OS time and the configured TZ for HA differ, the alert component will send one notification every second.

This change forces it to use the defined TZ, and with that, it only sends notifications when they are expected.

Related issue (if applicable): fixes #21860

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#9474

Example entry for configuration.yaml (if applicable):

homeassistant:
  time_zone: America/Los_Angeles

alert:
  garage_door:
    name: Garage is open
    done_message: Garage is closed
    entity_id: input_boolean.garage_door
    state: 'on'
    repeat: 5
    can_acknowledge: true
    skip_first: true
    notifiers:
      - pushbullet

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code does not interact with devices:

• Tests have been added to verify that the new code works.