Do not keep generated key in build folder

postinstall.cmake.in

 		OUTPUT_STRIP_TRAILING_WHITESPACE
 	)
 file(INSTALL "@CMAKE_CURRENT_BINARY_DIR@/${SECRET_FILE}" DESTINATION "/etc" FILE_PERMISSIONS OWNER_READ OWNER_WRITE)
+file(REMOVE "@CMAKE_CURRENT_BINARY_DIR@/${SECRET_FILE}")
 endif()