CI: manually specify mpich include path

Work around a bug with FindMPI.cmake not detecting the include path for
mpich.

contrib/utilities/run_clang_tidy.sh

@@ -43,7 +43,7 @@ echo "SRC-DIR=$SRC"
 
 # enable MPI (to get MPI warnings)
 # export compile commands (so that run-clang-tidy.py works)
-ARGS=("-D" "DEAL_II_WITH_MPI=ON" "-D" "CMAKE_EXPORT_COMPILE_COMMANDS=ON" "-D" "CMAKE_BUILD_TYPE=Debug" "$@")
+ARGS=("-DMPI_INCLUDE_DIRS=/usr/include/x86_64-linux-gnu/mpich" "-D" "DEAL_II_WITH_MPI=ON" "-D" "CMAKE_BUILD_TYPE=Debug" "$@")
 
 # for a list of checks, see /.clang-tidy
 cat "$SRC/.clang-tidy"