fix(forks): resolve poolOptions.<name>.isolate from forks options

[AriPerkkio]

Description

• Fixes issue where poolOptions.forks.isolate is not resolved correctly
• Minimal reproduction at https://stackblitz.com/edit/vitejs-vite-pchfh4?file=vite.config.ts

This is working fine when using workspaces:

vitest/packages/vitest/src/node/workspace.ts

Lines 344 to 348 in c79b3f1

	poolOptions: {
	forks: {
	singleFork: poolOptions?.forks?.singleFork ?? this.ctx.config.poolOptions?.forks?.singleFork ?? false,
	isolate: poolOptions?.forks?.isolate ?? isolate ?? this.ctx.config.poolOptions?.forks?.isolate ?? true,
	},

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