Bug: `formideploy archives` shouldn't require deploy validation.

For example, in `formidable.com` repo if we do `formideploy archives` in an uninstalled/non-built repo we get this:

```
[formideploy] Starting {"action":"archives"} with options {"port":5000,"limit":10,"start":"2021-04-06T14:49:38.241Z","dryrun":false,"production":false,"staging":false}
[formideploy][error] Error: Build output missing at: public
at validate (/Users/rye/scm/fmd/formideploy/lib/config/index.js:210:11)
at async archives (/Users/rye/scm/fmd/formideploy/lib/actions/archives.js:208:3)
at async main (/Users/rye/scm/fmd/formideploy/bin/formideploy.js:95:3)
```

Reproduce `formideploy archives` in an uninstalled or non-built repository, then inspect `lib/actions/archives.js` and the `validate` logic in `lib/config/index.js`. Confirm that the archives action can run without requiring the `public` build output, while preserving validation where it is still needed.

Scritto dal modello di indicizzazione a partire dal testo della issue.