Automatic toolchain upgrade to nightly-2025-08-21

[github-actions]

Update Rust toolchain from nightly-2025-08-20 to nightly-2025-08-21 without any other source changes.

[github-actions]

This is an automatically generated pull request. If any of the CI checks fail,
manual intervention is required. In such a case, review the changes at
https://github.com/rust-lang/rust from
rust-lang/rust@05f5a58 up to
rust-lang/rust@040a98a. The log
for this commit range is:

https://github.com/rust-lang/rust/commit/040a98af70 Auto merge of #144086 - clubby789:alloc-zeroed, r=nikic

rust-lang/rust@e8a792daf5 Auto merge of #145645 - Kobzol:uplift-fix, r=jieyouxu
rust-lang/rust@bec747418c Auto merge of #145348 - nnethercote:parse_token_tree-speedup-for-uom, r=petrochenkov
rust-lang/rust@22a86f8280 Auto merge of #145644 - jhpratt:rollup-ypo3zcd, r=jhpratt
rust-lang/rust@816f098464 Rollup merge of #145626 - folkertdev:prefetch-fallback, r=Amanieu
rust-lang/rust@ef22202db2 Rollup merge of #145623 - compiler-errors:pretty-async-name, r=wesleywiser
rust-lang/rust@6e6522980a Rollup merge of #145622 - tgross35:remove-builtins-patch, r=Mark-Simulacrum
rust-lang/rust@84c5d20fe7 Rollup merge of #145619 - joshtriplett:use-the-right-core, r=tgross35
rust-lang/rust@b0600b2326 Rollup merge of #145531 - sayantn:detect-apxf-avx10, r=Amanieu
rust-lang/rust@9261fcf159 Rollup merge of #145417 - a4lg:riscv-arch-platform-guide-ch2, r=Amanieu
rust-lang/rust@5a0c9386a2 Rollup merge of #145381 - Gnurou:int_lowest_highest_one, r=jhpratt
rust-lang/rust@a8feea8fc5 Rollup merge of #145359 - GuillaumeGomez:correctly-pick-search.js, r=lolbinarycat
rust-lang/rust@7b7ad4d4df Rollup merge of #145252 - shepmaster:demote-x86_64-apple-darwin-to-tier-2, r=Kobzol,madsmtm
rust-lang/rust@5fa33047a2 Rollup merge of #145006 - ginnyTheCat:docs-skip-until, r=ibraheemdev
rust-lang/rust@1e6df58e77 Rollup merge of #140794 - karolzwolak:allow-unused-doc-65464, r=davidtwco
rust-lang/rust@9a1ab5e402 Rollup merge of #140314 - lolbinarycat:rustdoc-js-scrape-examples-typecheck, r=notriddle
rust-lang/rust@a6d648fe79 Rollup merge of #139357 - miried:master, r=Amanieu
rust-lang/rust@f605b57042 Auto merge of #145601 - jieyouxu:rollup-t5mbqhc, r=jieyouxu

[tautschnig]

We are facing rust-lang/rust#145699. Waiting for details to be posted on that issue.

[zhassan-aws]

Closing in favor of #4316