Always using .bat files in Conan subprocess, not .ps1

[memsharded]

Changelog: Fix: Remove support for tools.env.virtualenv:powershell=True/False, deprecated more than 1 year ago
Changelog: Fix: Use always .bat files for Conan subprocesses (self.run()), instead of .ps1, that are only necessary for users running natively in powershell for local development.
Docs: https://github.com/conan-io/docs/pull/XXXX

Close #11353

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[AbrilRBS]

approving dropping the boolean value support

[memsharded]

This is something that could be considered breaking, because if the recipe hardcodes somewhere that is adding some .ps1, then it becomes necessary to explicitly tell the recipe to use that .ps1, because by default now we made to use the conanbuild.bat always.