[ci] Roll pinned nightly toolchain (#2868)

Author: google-pr-creation-bot

Cargo.toml

@@ -85,7 +85,7 @@ no-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 = "1.92.0"
-pinned-nightly = "nightly-2025-12-21"
+pinned-nightly = "nightly-2026-01-04"
 
 [package.metadata.docs.rs]
 all-features = true