The script s_clang_tidy does not use the toolchain binary:

if test -z "$tidy" &&
    test -x /usr/lib/llvm-3.9/bin/clang-tidy; then
    tidy="/usr/lib/llvm-3.9/bin/clang-tidy"
fi

We should update it to use the stable version that is currently located here: /opt/mongodbtoolchain/v4/bin/clang-tidy