[Refactor] Simplify mkTmpDir in cli-kit

[gonzaloriestra]

This PR simplifies the mkTmpDir function in @shopify/cli-kit.

• Replaces os.tmpdir() with systemTempDir to match other temporary directory utilities.
• Uses joinPath consistently for path joining.
• Removes an unnecessary intermediate variable and returns the promise directly.

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PR created automatically by Jules for task 11967171989926349973 started by @gonzaloriestra

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