[browser] mocking + cacheDir fails for some cacheDirs

Describe the bug

When using vitest browser mode, mocking does not work if cacheDir is set to a subparent directory.
This DOES work properly in node mode.

Reproduction

https://github.com/meesvandongen/vitest-repro

npm run test:child:browser // success
npm run test:parent:browser // fails
npm run test:parent:node // success

System Info

browser = chrome, playwright; did not test with other setups

  System:
    OS: macOS 15.1
    CPU: (16) arm64 Apple M3 Max
    Memory: 7.45 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.11.0 - ~/.nvm/versions/node/v22.11.0/bin/node
    npm: 10.9.0 - ~/.nvm/versions/node/v22.11.0/bin/npm
    pnpm: 9.4.0 - /opt/homebrew/bin/pnpm
    bun: 1.1.31 - ~/.bun/bin/bun
  Browsers:
    Chrome: 131.0.6778.69
    Edge: 131.0.2903.48
    Safari: 18.1
  npmPackages:
    @vitest/browser: ^2.1.5 => 2.1.5

Used Package Manager

npm

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• Read the Contributing Guidelines.
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• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

Start with the linked minimal reproduction and run npm run test🧒browser, npm run test:parent:browser, and npm run test:parent:node. Compare the browser-mode parent and child cases around cacheDir and mocking; done means the parent browser command succeeds while the existing child-browser and parent-node cases continue to pass.