Specified git branch for google benchmark fetch

The default branch was renamed from "master" to "main", which breaks the cloning process.

benchmarks/CMakeLists.txt

@@ -11,6 +11,7 @@ include(FetchContent)
 FetchContent_Declare(
     benchmark
     GIT_REPOSITORY https://github.com/google/benchmark.git
+    GIT_TAG origin/main
     GIT_SHALLOW TRUE
 )