Properly set agent_win_install_args in all cases

[KSerrania]

What does this PR do?

Initializes agent_win_install_args to win_install_args in all cases.

Removes useless line that sets agent_win_install_args to itself.

Motivation

agent_win_install_args is used in pkg-windows.yml and pkg-windows-opts.yml. It is supposed to be set in pkg-windows-opts.yml, but all tasks that set it do it only under certain conditions.

On a default install, the role crashes with:

{"msg": "The task includes an option with an undefined variable. The error was: 'agent_win_install_args' is undefined. 'agent_win_install_args' is undefined\n\nThe error appears to be in '/Users/kylian.serrania/.ansible/collections/ansible_collections/datadog/dd/roles/agent/tasks/pkg-windows-opts.yml': line 84, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: Set Test\n  ^ here\n"}