rustc: ensure standard library docs are installed

Fixes #405914

Upstream's installation script installs the `docs` component
for every target that is built.
Unfortunately, installing the docs for a target removes the
previous target's docs, ultimately leaving only the docs
of the last target in the list.
In our case, that is a `no_std` target, for which no `alloc`
or `std` docs are built, thus leaving `rustc.doc` without any
standard library docs.
Moving stdenv's `targetPlatform` to the end of the list ensures
we get standard library docs.

0 files changed, 0 insertions, 0 deletions