Send and Sync impls of `::env` types depend on implementation

[panicbit]

::env::{Args,ArgsOs,Vars,VarsOs} depend entirely on the platform impls in ::sys for their negative Send/Sync impls.
Implementations of those types currently opt out of Send and Sync, so the user facing types should probably explicitly opt out of Send/Sync to avoid accidental inconsistencies between platforms.