Makefile: add _artifacts to MKDIR_TARGETS again

It seems that variable expansion in Makefile targets is not done recursively.

Signed-off-by: Norio Nomura <norio.nomura@gmail.com>

Makefile

@@ -203,8 +203,6 @@ _output/share/doc/lima/%: % | _output/share/doc/lima
 
 MKDIR_TARGETS += _output/share/doc/lima
 
-$(MKDIR_TARGETS):
-	mkdir -p $@
 
 .PHONY: create-links-in-documents
 create-links-in-documents: _output/share/doc/lima/templates _output/share/doc/lima/examples
@@ -318,8 +316,7 @@ _artifacts/lima-%.tar.gz: | _artifacts
 	$(ENVS) make clean binaries
 	$(TAR) -C _output/ -czvf $@ ./
 
-_artifacts:
-	mkdir -p $@ 
+MKDIR_TARGETS += _artifacts
 
 .PHONY: artifacts-misc
 artifacts-misc: | _artifacts
@@ -331,3 +328,8 @@ codesign:
 ifeq ($(GOOS),darwin)
 	codesign --entitlements vz.entitlements -s - ./_output/bin/limactl
 endif
+
+# This target must be placed after any changes to the `MKDIR_TARGETS` variable.
+# It seems that variable expansion in Makefile targets is not done recursively.
+$(MKDIR_TARGETS):
+	mkdir -p $@