As of late, `final: prev: …` for overlays has become more prevalent in
newer code. This is also exhibited in some code (presumably added
recently) in stdenv. This change is not about any merits or demerits of
any naming convention, but rather aims to make the nomenclature in
stdenv bootstrapping consistent to lessen confusion.
I've chosen to stick to `self: super: …` convention because:
1. It is more common in the code as it stands.
2. Using `final: prev: …` makes the code more confusing, als it causes
`prev` to be in scope alongside `prevStage`. `prevStage` actually
bears no relation to `prev` even though their naming suggests it,
making it easy to confuse them (the former is the (final) package set
of the previous stage while the latter is just the `prev`/`super` of
the overlay “chaining” on a completely fresh package set, i.e. `prev`
doesn't even relate to the previous stage's `overrides` argument).
This change also corrects a naming error in stdenv/native which had no
effect, as the variables were unused.
0198f43efb