Skip cloning registry when loaded from cache

[omus]

When the registry input is defined we always clone that registry even if the registry is already present:

add-julia-registry/main.js

Lines 42 to 51 in 2baaa59

	await exec.exec(`git clone git@github.com:${registry}.git ${tmpdir}`);
	const meta = toml.parse(fs.readFileSync(path.join(tmpdir, "Registry.toml")));
	const name = meta.name || registry.split("/")[1];
	const depot = process.env.JULIA_DEPOT_PATH || path.join(home, ".julia");
	const dest = path.join(depot, "registries", name);
	if (fs.existsSync(dest)) {
	tmpdirCleanup();
	} else {
	fs.moveSync(tmpdir, dest);
	}

It would be nice to avoid this unnecessary clone when using cached registries