Replace rsync with tar-pipe copy in appstore packaging

The self-hosted Gitea runner's image has no rsync installed. tar is
a core utility present on essentially any Linux base image, and its
--exclude on a directory prevents descending into it at all, which
also avoids the build/ output dir (itself inside the source tree)
being copied into itself.

Makefile

@@ -6,25 +6,26 @@ appstore_artifact_directory=$(build_directory)/artifacts/appstore
 .PHONY: appstore
 appstore: clean
 	mkdir -p $(appstore_build_directory) $(appstore_artifact_directory)
-	rsync -av . $(appstore_build_directory) \
-		--exclude=/.git \
-		--exclude=/.gitea \
-		--exclude=/.gitignore \
-		--exclude=/composer.json \
-		--exclude=/composer.lock \
-		--exclude=/package.json \
-		--exclude=/package-lock.json \
-		--exclude=/node_modules \
-		--exclude=/vite.config.js \
-		--exclude=/src \
-		--exclude=/tests \
-		--exclude=/vendor \
-		--exclude=/build \
-		--exclude=/.php-cs-fixer.dist.php \
-		--exclude=/.php-cs-fixer.cache \
-		--exclude=/phpunit.xml \
-		--exclude=/README.md \
-		--exclude=/Makefile
+	tar -cf - \
+		--exclude=./.git \
+		--exclude=./.gitea \
+		--exclude=./.gitignore \
+		--exclude=./composer.json \
+		--exclude=./composer.lock \
+		--exclude=./package.json \
+		--exclude=./package-lock.json \
+		--exclude=./node_modules \
+		--exclude=./vite.config.js \
+		--exclude=./src \
+		--exclude=./tests \
+		--exclude=./vendor \
+		--exclude=./build \
+		--exclude=./.php-cs-fixer.dist.php \
+		--exclude=./.php-cs-fixer.cache \
+		--exclude=./phpunit.xml \
+		--exclude=./README.md \
+		--exclude=./Makefile \
+		. | tar -xf - -C $(appstore_build_directory)
 	tar -czf $(appstore_artifact_directory)/$(app_name).tar.gz -C $(build_directory)/appstore $(app_name)
 
 .PHONY: clean