[ci] Roll pinned nightly toolchain (#1272)

Author: google-pr-creation-bot

Cargo.toml

@@ -46,7 +46,7 @@ zerocopy-panic-in-const = "1.57.0"
 [package.metadata.ci]
 # The versions of the stable and nightly compiler toolchains to use in CI.
 pinned-stable = "1.78.0"
-pinned-nightly = "nightly-2024-05-14"
+pinned-nightly = "nightly-2024-05-15"
 
 [package.metadata.docs.rs]
 all-features = true