update-desktop-database is called from the pkg_postinst stage instead.

--- openclonk-release-6.0-src.orig/CMakeLists.txt
+++ openclonk-release-6.0-src/CMakeLists.txt
@@ -1564,7 +1564,8 @@
 	# Install new files
 	install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/openclonk.desktop DESTINATION "${INSTALL_DATAROOTDIR}/applications")
 	# Update the MIME cache -- this makes the clonk:// protocol handler actually work
-	install(CODE "execute_process(COMMAND update-desktop-database)")
+	# --> ebuild, pkg_postinst stage
+	#install(CODE "execute_process(COMMAND update-desktop-database)")
 
 	# Install binaries
 	install(TARGETS openclonk DESTINATION "${INSTALL_GAMES_BINDIR}")