Fix a cross-device mv error by using a tmp directory inside document root

[adamziel]

What is this PR doing?

Solves the error discovered by @sejas caused by calling php.mv(fromPath, toPath) when fromPath is a mounted local directory and toPath is a MEMFS directory. Emscripten doesn't support such a scenario:

Proceeding without the Notification plugin. Could not install it in wp-admin. The original error was: Error: Could not move "/tmp/assets/Notification/notification": Cross-device link.
Error: Could not move "/tmp/assets/Notification/notification": Cross-device link.
    at descriptor.value (/playground-tools/node_modules/@php-wasm/node/index.cjs:67481:17)

The solution proposed in this PR replaces a /tmp directory with a randomly-named temporary directory inside wp-content. /tmp doesn't necessarily point to a system temp directory and needs to be revisited anyway. Whether we should use a temporary directory inside wp-content is another matter, but that part may be revisited once the recursive cp feature is added.

Testing instructions

• Confirm all the CI checks pass.
• Build the project (npm run build)
• Link the packages as described in Add pooling and request limiting to prevent memory leaks playground-tools#110
• Run the test plan provided in WIP wp-now: update blueprints dependencies playground-tools#116

[adamziel]

I just confirmed this fixes Blueprints in wp-now cc @sejas: