fix(core): Initialize application only once

[KKSzymanowski]

Fixes #12006

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

Issue Number: #12006

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

[coveralls]

Pull Request Test Coverage Report for Build 3ef0d4aa-78de-4726-bebb-441a4fa97c44

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.864%

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Totals
Change from base Build cbbb8d78-86c6-4df5-9605-650f7c1be038:	0.0%
Covered Lines:	6364
Relevant Lines:	6853

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💛 - Coveralls

[kamilmysliwiec]

lgtm