fix(experiments): use getters to prevent toc/tou limitations

[rektdeckard]

Previously, the value of experiment flags were read from the environment during the init() function, making it impossible to programmatically enable or disable flags when using task as a package (#2014). This patch makes all experiments getters that read from the environment at time of access, not at time of initialization.

A more flexible solution might be to expose experiments to the Executor and allow values to be overridden directly.