fix(@angular-devkit/build-angular): use correct type for extraEntryPoints

[ocombe]

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• 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
• Documentation content changes
• Other... Please describe:

What is the current behavior?

The type is incorrect

Issue Number: N/A

What is the new behavior?

The type is correct

Does this PR introduce a breaking change?

• Yes
• No

Other information

[alan-agius4]

LGTM, Thanks

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