Specified git branch for google benchmark fetch in benchmark test

[grafail]

The default branch in https://github.com/google/benchmark was renamed from "master" to "main", which breaks the cloning process during building the benchmark:

CMakeLists Original Source:

FetchContent_Declare(
    benchmark
    GIT_REPOSITORY https://github.com/google/benchmark.git
    GIT_SHALLOW TRUE
)

GIT_TAG origin/main was added to the command.

---

Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header file single_include/nlohmann/json.hpp. The whole process is described here.

Please don't

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[nlohmann]

Looks good to me.

[coveralls]

Coverage remained the same at 99.982% when pulling b7e493e on turintech:develop into 6471a63 on nlohmann:develop.

[nlohmann]

Please update the branch - I just merged #2817 to fix the CI.