Makefile: add pyexternals target

When running `make external` not all externals are created right
now, only the go ones. This commit ensures all are available inside
the `./external` dir.

Makefile

@@ -125,6 +125,7 @@ $(EXTERNAL_GO_EXECUTABLES_FULLPATH): | $(EXTERNAL_DIR)
 # sources instead of pulling of the network
 .PHONY: external
 external: $(EXTERNAL_GO_EXECUTABLES_FULLPATH)
+	ln -sf $(VENV_DIR)/bin/osbuild-* $(EXTERNAL_DIR)/
 
 clean: ## clean all build artifacts
 	rm -rf $(VENV_DIR)