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

Merge branch 'feature/install-scripts-executable' into 'master'

ref:core/dune-common\> 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\...).

See merge request [!812]

  [!812]: gitlab.dune-project.org/core/dune-common/merge_requests/812

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
+)