[!828] [!812] [cmake][bugfix] Use install(PROGRAMS...) signature to install executable scripts

Merge branch 'cherry-pick-cc5ed571' into 'releases/2.6'

ref:core/dune-common\> 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]]

(cherry picked from commit cc5ed571)

ee4cef6d \[cmake\]\[bugfix\] Use install(PROGRAMS\...) signature to install
executable scripts

See merge request [!828]

  [!812]: gitlab.dune-project.org/core/dune-common/merge_requests/812
  [!828]: gitlab.dune-project.org/core/dune-common/merge_requests/828