The `core.GetPlatforms` function ignores the `all` field showing manually installed cores

The `core.GetPlatforms` function is not working as intended right now but the logic of this function is good.
It ignores the `All` field in the rpc.PlatformListRequest showing the manually installed cores even if the `All` field is set to false.
see https://github.com/arduino/arduino-cli/pull/1509#discussion_r730758844