Super cache - fix tempnam system directory warning

[donnchawp]

When WPSC is first installed it tries to create a temporary file when writing to the config file. As the cache directory hasn't been set up correctly yet, it can't, so it creates it in the system tmp directory. That results in a PHP warning. This PR fixes that by creating the cache directory first.

Fixes #

Proposed changes:

• Call wp_cache_verify_cache_dir() before writing to the config file so the cache directory is there already.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Activate the plugin and you will see a warning, "tempnam(): file created in the system's temporary directory in..."
• Apply this PR and deactivate and activate it and that warning won't show.

[github-actions]

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Super Cache plugin:

• Next scheduled release: July 2, 2024.
• Scheduled code freeze: June 24, 2024.

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[dilirity]

No more notices, nice!