Update Platformio to version 3.6.6, resolves T7820
-Project Generator: fixed a warning "Property !!! WARNING !!! is not allowed" for VSCode. -Fixed an issue when PlatformIO Build System does not pick up "mbed_lib.json" files from libraries. -Fixed an error with conflicting declaration of a prototype (Arduino sketch preprocessor). -Fixed "FileExistsError" when platformio ci command is used in pair with --keep-build-dir option. -Fixed an issue with incorrect order of project "include" and "src" paths in CPPPATH.
Test Plan: Installed and started without errors/problems
Reviewers: Triage Team, Girtablulu, DataDrake
Reviewed By: Triage Team, DataDrake
Subscribers: DataDrake, Girtablulu
Maniphest Tasks: T7820
Differential Revision: https://dev.getsol.us/D5945