Tweak src/Makefile for subsequent rebuilds

Fixes an issue rebuilding when `dist` has been removed.

src/Makefile

@@ -19,11 +19,10 @@ BASE_URL ?= "./"
 PKG_BASE_URL ?= "https://repo.r-wasm.org"
 
 HTML_DIST = $(addprefix $(DIST)/,$(HTML_TEMPLATES))
-$(DIST): $(TS_SOURCES) $(HTML_DIST) \
+$(DIST)/webr.mjs: $(TS_SOURCES) $(HTML_DIST) \
   Makefile webR/config.ts node_modules esbuild.ts
 	cp $(DIST)/$(HTML_INDEX) $(DIST)/index.html
 	npm run build
-	touch $@
 
 $(DIST)/%.html: templates/%.html
 	sed -e "s|@@BASE_URL@@|$(BASE_URL)|" \