Fix lts cannot be specified by node-version at node/install

[yasuoza]

SEMVER Update Type:

• Major
• Minor
• Patch

Description:

Document says

 To install the latest LTS version, set the version to `lts`.

https://circleci.com/developer/ja/orbs/orb/circleci/node?version=5.0.0#commands-install

But, node-version: lts fails with following error.

Version 'lts' not found - try `nvm ls-remote` to browse available versions.

Exited with code exit status 3

This is causes by script does not handle node-version: lts well.
This PR fixes the issue.

Motivation:

I want to set and fix node version lts, but with version 5.0.0, I can't.

Checklist:

• All new jobs, commands, executors, parameters have descriptions.
• Usage Example version numbers have been updated.
• Changelog has been updated.

[Jaryt]

Hi! Thank you for bringing this up. I believe this is actually due to me missing that documentation that was leftover from the change to default to lts rather than latest. To use 'lts', the node-version string would remain empty. I'm trying to decide whether it's worth adding this check in case someone does set the string to lts.

Regardless, I'll need to fix that piece of documentation.

[Jaryt]

Updated the comment and merged since it wouldn't hurt if someone explicitly defined the version as lts.