appimage: fix possible issue with the appimagetool binary (#14717)

Zero-out AppImage magic bytes from the ELF header extended ABI version so
binfmt+qemu can be used (e.g. when executed from `docker run --platform …`).

Cf.:
- https://github.com/AppImage/AppImageKit/issues/1056
- https://github.com/AppImage/AppImageSpec/blob/master/draft.md#type-2-image-format
- https://en.wikipedia.org/wiki/Executable_and_Linkable_Format#:~:text=EI_ABIVERSION

make/appimage.mk

@@ -14,7 +14,16 @@ plugins/timesync.koplugin
 $(filter-out tools/trace_require.lua tools/wbuilder.lua,$(wildcard tools/*))
 endef
 
-update: all
+appimagetool $(APPIMAGETOOL):
+	wget -O $(APPIMAGETOOL).part '$(APPIMAGETOOL_URL)'
+	# Zero-out AppImage magic bytes from the ELF header extended ABI version so
+	# binfmt+qemu can be used (e.g. when executed from `docker run --platform …`).
+	# Cf. https://github.com/AppImage/AppImageKit/issues/1056.
+	printf '\0\0\0' | dd conv=notrunc obs=1 seek=8 of=$(APPIMAGETOOL).part
+	chmod +x ./$(APPIMAGETOOL).part
+	mv $(APPIMAGETOOL).part $(APPIMAGETOOL)
+
+update: all $(APPIMAGETOOL)
 	cd $(INSTALL_DIR)/koreader && '$(abspath tools/mkrelease.sh)' ../appimage/ . $(release_excludes)
 	cp $(APPIMAGE_DIR)/{AppRun,koreader.desktop} resources/koreader.png $(INSTALL_DIR)/appimage/
 	sed -e 's/%%VERSION%%/$(VERSION)/' -e 's/%%DATE%%/$(RELEASE_DATE)/' $(PLATFORM_DIR)/common/koreader.metainfo.xml >$(INSTALL_DIR)/appimage/koreader.appdata.xml
@@ -24,13 +33,9 @@ ifeq (,$(wildcard $(UBUNTU_LIBBSD)))
 	echo 'WARNING: not bundling missing $(UBUNTU_LIBBSD)' 1>&2
 else
 	cp $(UBUNTU_LIBBSD) $(INSTALL_DIR)/appimage/libs/
-endif
-ifeq (,$(wildcard $(APPIMAGETOOL)))
-	# Download appimagetool.
-	wget '$(APPIMAGETOOL_URL)'
-	chmod a+x ./$(APPIMAGETOOL)
 endif
 	# Generate AppImage.
 	ARCH='$(APPIMAGE_ARCH)' ./$(APPIMAGETOOL) --appimage-extract-and-run $(INSTALL_DIR)/appimage $(KOREADER_APPIMAGE)
 
-PHONY += update
+PHONY += appimagetool update
+SOUND += $(APPIMAGETOOL)