feat(cli): allow skip-install for add

[z3bra5hax]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: 1809

What is the new behavior?

Allows the use of skip-install option flag for add actions

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

No current convention exists within this repository for unit testing actions/commands.
Availability of flag should be updated in general docs.

[kamilmysliwiec]

lgtm