[cmake][bugfix] Use install(PROGRAMS...) signature to install executable scripts

CMake does not preserve file permissions when installing files.
In order to get executable scripts installed, the install(PROGRAMS...)
signature needs to be used instead of install(FILES...).

cmake/scripts/CMakeLists.txt

+# Install non-executable scripts
 install(FILES
   conf.py.in
   CreateDoxyFile.cmake
   envdetect.py
-  extract_cmake_data.py
   FinalizeHeadercheck.cmake
   index.rst.in
   InstallFile.cmake
   main77.cc.in
   module_library.cc.in
   pyversion.py
-  run-in-dune-env.sh.in
   RunDoxygen.cmake
   sphinx_cmake_dune.py
   DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/dune/cmake/scripts)
+
+# Install executable programs
+install(PROGRAMS
+  extract_cmake_data.py
+  run-in-dune-env.sh.in
+  DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/dune/cmake/scripts
+)