Try to avoid the last rustc rebuild

Author: Kobzol

src/ci/stage-build.py

@@ -798,12 +798,15 @@ def execute_build_pipeline(timer: Timer, pipeline: Pipeline, final_build_args: L
                     "--llvm-profile-use",
                     pipeline.llvm_profile_merged_file(),
                     "--llvm-bolt-profile-generate",
+                    "--rust-profile-use",
+                    pipeline.rustc_profile_merged_file()
                 ])
                 record_metrics(pipeline, rustc_build)
 
             with stage3.section("Gather profiles"):
                 gather_llvm_bolt_profiles(pipeline)
 
+        # LLVM is not being cleared here, we want to reuse the previous build
         print_free_disk_space(pipeline)
         final_build_args += [
             "--llvm-bolt-profile-use",