Move to xz dist tarballs

For consistency with the already xz'ed node tarball.

Rename the oddly named `dist-gzip` target to the standard `dist`.

Cherry-picked from starter-kit commit 4ca75f143c

Makefile

@@ -5,7 +5,7 @@ ifeq ($(TEST_OS),)
 TEST_OS = rhel-x
 endif
 export TEST_OS
-TARFILE=cockpit-$(PACKAGE_NAME)-$(VERSION).tar.gz
+TARFILE=cockpit-$(PACKAGE_NAME)-$(VERSION).tar.xz
 RPMFILE=$(shell rpmspec -D"VERSION $(VERSION)" -q cockpit-session-recording.spec.in).rpm
 VM_IMAGE=$(CURDIR)/test/images/$(TEST_OS)
 # stamp file to check if/when npm install ran
@@ -88,7 +88,7 @@ devel-install: $(WEBPACK_TEST)
 	mkdir -p ~/.local/share/cockpit
 	ln -s `pwd`/dist ~/.local/share/cockpit/$(PACKAGE_NAME)
 
-dist-gzip: $(TARFILE)
+dist: $(TARFILE)
 
 # when building a distribution tarball, call webpack with a 'production' environment
 # we don't ship node_modules for license and compactness reasons; we ship a
@@ -100,7 +100,7 @@ $(TARFILE): $(WEBPACK_TEST) cockpit-$(PACKAGE_NAME).spec
 	mv node_modules node_modules.release
 	touch -r package.json $(NODE_MODULES_TEST)
 	touch dist/*
-	tar czf cockpit-$(PACKAGE_NAME)-$(VERSION).tar.gz --transform 's,^,cockpit-$(PACKAGE_NAME)/,' \
+	tar --xz -cf cockpit-$(PACKAGE_NAME)-$(VERSION).tar.xz --transform 's,^,cockpit-$(PACKAGE_NAME)/,' \
 		--exclude cockpit-$(PACKAGE_NAME).spec.in \
 		$$(git ls-files) package-lock.json cockpit-$(PACKAGE_NAME).spec dist/
 	mv node_modules.release node_modules
@@ -175,4 +175,4 @@ $(NODE_MODULES_TEST): package.json
 	env -u NODE_ENV npm install
 	env -u NODE_ENV npm prune
 
-.PHONY: all clean install devel-install dist-gzip srpm rpm check vm update-po
+.PHONY: all clean install devel-install dist srpm rpm check vm update-po