Enable task run setting to be able reinitialise

[khalidmammadov]

When task is executed using "StandardTaskRunner._start_by_exec()" (when fork is not available) it's using old DONOT_MODIFY_HANDLERS config variable loaded during settings.py load and not the one from externally supplied --cfg-path param

TL;DR

When "tasks run" command is executed it then internally re-executed as below command using external process (non fork when it's not available).
[2021-11-16 15:28:16,818] {base_task_runner.py:144} INFO - Running: ['airflow', 'tasks', 'run', 'test_logging_dag', 'test_task', 'test_run', '--job-id', '102', '--raw', '--subdir', 'DAGS_FOLDER', '--cfg-path', '/tmp/tmpfv4cj9py', '--error-file', '/tmp/tmphjtaj5gi']

Here '--cfg-path', '/tmp/tmpfv4cj9py' is a copy of active config settings of parent process. When command is executed settings.py file is loaded as one of the first things but external config (/tmp/tmpfv4cj9py) is loaded later within task_command.py->def task_run.
And when it does load it does not re-initialize this setting.
This causes DONOT_MODIFY_HANDLERS to be initialized to a default value rather than one that is defined in the /tmp/tmpfv4cj9py
The PR fixes it by moving initialization into settings.py/configure_vars function where it can be called when needed.

PS:
I think this issue is not limited to this variable but all other similar GLOBAL variables that is initialized during settings.py load.
But I think we need to check each of them separately before making similar changes.

#19755 linked to this which I closed in favor this.

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[potiuk]

We REALLY have to redo the way logging is handled.