fix: do not hardcode npx in shadcn-ui:add executor

[gurisko]

Fixes #245

[nx-cloud]

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✅ Successfully ran 6 targets

• nx run-many --target=e2e --projects=translations-e2e,react-email-e2e,shadcn-ui-e2e,strapi-e2e --output-style=stream
• nx run-many --target=e2e --projects=gcp-deployment-manager-e2e,gcp-functions-e2e,gcp-secrets-e2e --output-style=stream
• nx run-many --target=build --projects=gcp-deployment-manager,firebase-hosting,gcp-cloud-run,gcp-functions,gcp-secrets,gcp-storage,e2e-runner,core --output-style=stream --prod
• nx run-many --target=build --projects=translations,react-email,playwright,shadcn-ui,terraform,pulumi,strapi,vercel --output-style=stream --prod
• nx run-many --target=test --projects=gcp-deployment-manager,firebase-hosting,gcp-cloud-run,gcp-functions,translations,gcp-secrets,gcp-storage,react-email,e2e-runner,playwright,shadcn-ui,terraform,pulumi,strapi,vercel,core --output-style=stream --parallel=4
• nx run-many --target=version --projects=gcp-deployment-manager,firebase-hosting,gcp-cloud-run,gcp-functions,translations,gcp-secrets,gcp-storage,react-email,e2e-runner,playwright,shadcn-ui,terraform,pulumi,strapi,vercel,core --output-style=stream

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