Bump download-ci-llvm stamp

This will ensure the optimized LLVM is used for local builds after rebasing
immediately, rather than needing to wait for a future LLVM bump.

Author: Mark-Simulacrum

src/bootstrap/download-ci-llvm-stamp

@@ -1,4 +1,4 @@
 Change this file to make users of the `download-ci-llvm` configuration download
 a new version of LLVM from CI, even if the LLVM submodule hasn’t changed.
 
-Last change is for: https://github.com/rust-lang/rust/pull/80932
+Last change is for: https://github.com/rust-lang/rust/pull/88069