fix(cli): Update start.command.ts to allowUnknownOption

[zackdotcomputer]

Commander requires an explicit call to allowUnknownOption to allow and not error on options it does not recognize. This causes an issue with this functionality because any user-defined flags are blocked at the command level.

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) - no because the bug is in the usage of commander
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] 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?

This is a proposal for a fix to restore the behavior of forwarding flags on to the nest start command. Currently, commander fails any calls with --xxxx flags it doesn't recognize, so that functionality doesn't work.

Issue Number: #1844

What is the new behavior?

This adds the allowUnknownOption behavior so commander will no longer fail calls with --xxxx flags it does not recognize.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Refer to initial PR on this work: #1847

[kamilmysliwiec]

lgtm