fix(@angular-devkit/build-angular): prevent race condition in setting up sass worker pool

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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?

Multiple calls to the SassStylesheetLanguage's process function can lead to setting up multiple sass worker pools, only one of which is cleaned up after the esbuild application build.

The result is ng build hanging forever waiting for the workers to exit.

Issue Number: N/A

What is the new behavior?

Prevent the race condition, thus setting up only one worker pool and allowing the process to exit gracefully.

Does this PR introduce a breaking change?

• Yes
• No

Other information

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