Synchronously copy files when watchAssets: false

[tkow]

PR Checklist

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• 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?

[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?

Even if watchAssets flag is false, the running nest start process can't exit from entry file because chokidar watches files and entry script runs as another child process when using webpack.
The reason why is internal runWebpack method doesn't call assetsManager.closeWatchers() when using nest start, but nest build does. At first, simply I tried to call assetsManager.closeWatchers() after onSuccess in compiler/webpack but, it can't listen to complete all entry file's processes. Additionally, current chokidar watcher can't detect whether all files matched by glob patterns is copied, so I think it would be better to process them synchronously when watchAssets: false is specified.

Issue Number: N/A

What is the new behavior?

Copy files process runs Synchronously.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information