fix(nextjs): Ensure build-custom-server is only added once to nx.json

[ndcunningham]

This PR fixes the issue of when creating a new Next.js project with the --custom-server flag it will be only added once to nx.json's cacheableOperations

closes: #18957

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Name	Status	Preview	Comments	Updated (UTC)
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✅ Successfully ran 8 targets

• nx affected --target=e2e --base=ede4b96e8d49f46ae8e3d5fffffd1ea309406905 --head=025899cc562ae06cef30fcecf87cb972add58ea3 --parallel=1
• nx affected --target=build --base=ede4b96e8d49f46ae8e3d5fffffd1ea309406905 --head=025899cc562ae06cef30fcecf87cb972add58ea3 --parallel=3
• nx affected --target=test --base=ede4b96e8d49f46ae8e3d5fffffd1ea309406905 --head=025899cc562ae06cef30fcecf87cb972add58ea3 --parallel=1
• nx affected --target=lint --base=ede4b96e8d49f46ae8e3d5fffffd1ea309406905 --head=025899cc562ae06cef30fcecf87cb972add58ea3 --parallel=3
• nx run-many -t check-imports check-commit check-lock-files check-codeowners documentation --parallel=1 --no-dte
• nx-cloud record -- nx format:check --base=ede4b96e8d49f46ae8e3d5fffffd1ea309406905 --head=025899cc562ae06cef30fcecf87cb972add58ea3
• nx documentation --no-dte
• nx affected -t e2e-macos --parallel=1 --base=ede4b96e8d49f46ae8e3d5fffffd1ea309406905 --head=025899cc562ae06cef30fcecf87cb972add58ea3

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