Add process.env.__NEXT_PPR to build config (#59253)
Adds process.env.__NEXT_PPR as static flag that it gets inlined by
DefinePlugin during build.
This matches what we do for other experiments, but I do wonder, why do
we bother with process.env at all instead of a global __NEXT_PPR symbol?
I'll leave that for later discussion.
Closes NEXT-1792
