[ci] Roll pinned nightly toolchain (#2687)

Author: google-pr-creation-bot

Cargo.toml

@@ -60,7 +60,7 @@ zerocopy-panic-in-const-and-vec-try-reserve-1-57-0 = "1.57.0"
 [package.metadata.ci]
 # The versions of the stable and nightly compiler toolchains to use in CI.
 pinned-stable = "1.89.0"
-pinned-nightly = "nightly-2025-08-20"
+pinned-nightly = "nightly-2025-08-21"
 
 [package.metadata.docs.rs]
 all-features = true

tests/ui-nightly/try_transmute_mut-size-increase.stderr

@@ -4,7 +4,7 @@ warning: unused import: `util::AU16`
 13 | use util::AU16;
    |     ^^^^^^^^^^
    |
-   = note: `#[warn(unused_imports)]` on by default
+   = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default
 
 error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
   --> tests/ui-nightly/try_transmute_mut-size-increase.rs:20:50

zerocopy-derive/tests/ui-nightly/enum.stderr

@@ -297,7 +297,7 @@ error[E0566]: conflicting representation hints
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #68585 <https://github.com/rust-lang/rust/issues/68585>
-   = note: `#[deny(conflicting_repr_hints)]` on by default
+   = note: `#[deny(conflicting_repr_hints)]` (part of `#[deny(future_incompatible)]`) on by default
 
 error[E0277]: the trait bound `UnsafeCell<()>: Immutable` is not satisfied
   --> tests/ui-nightly/enum.rs:51:10

zerocopy-derive/tests/ui-nightly/late_compile_pass.stderr

@@ -4,7 +4,7 @@ warning: unused import: `zerocopy::KnownLayout`
 15 | use zerocopy::KnownLayout;
    |     ^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: `#[warn(unused_imports)]` on by default
+   = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default
 
 error[E0277]: the trait bound `NotZerocopy: zerocopy::TryFromBytes` is not satisfied
   --> tests/ui-nightly/late_compile_pass.rs:29:10