Ensure the web asset dir is only checked during copy

[imhoffd]

The existence of the web asset directory should only be checked when running copy/sync, not during add or other commands.

The current behavior can lead to this error:

❯ npx cap add
[error] Could not find the web assets directory: /Users/dan/git/capacitor-testapp/build.
        Please create it and make sure it has an index.html file. You can change the path of this directory in
        capacitor.config.ts (webDir option). You may need to compile the web assets for your app (typically npm run
        build).
        More info: https://capacitorjs.com/docs/v3/basics/workflow#sync-your-project

Also see ionic-team/ionic-cli#4510

[imhoffd]

We're good. 💯

Related PR: #4215

[ionitron-bot]

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