build: make macOS HOST in download-osx generic

This was missed in bitcoin#20419, and the update before that, so just
make this un-versioned so that we don't have to worry about it.
This is fine, because it's just for downloading sources.

depends/Makefile

@@ -265,7 +265,7 @@ install: check-packages $(host_prefix)/share/config.site
 download-one: check-sources $(all_sources)
 
 download-osx:
-	@$(MAKE) -s HOST=x86_64-apple-darwin14 download-one
+	@$(MAKE) -s HOST=x86_64-apple-darwin download-one
 download-linux:
 	@$(MAKE) -s HOST=x86_64-unknown-linux-gnu download-one
 download-win: