[Refactor] Simplify mkTmpDir in cli-kit

Refactor `mkTmpDir` in `packages/cli-kit/src/public/node/fs.ts` to use `systemTempDir` and `join` for consistency with other temporary directory functions like `tempDirectory` and `inTemporaryDirectory`. Simplified the implementation by returning the `fsMkdtemp` promise directly and removing an unnecessary intermediate variable.

Author: gonzaloriestra

.jules/refactor.md

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+## 2025-05-20 - Use systemTempDir instead of os.tmpdir()
+**Smell:** Direct usage of `os.tmpdir()` in some filesystem utilities.
+**Learning:** The codebase provides `systemTempDir` in `@shopify/cli-kit/src/private/node/temp-dir.ts` which resolves the realpath of the temporary directory. This is critical for macOS (handling `/var` vs `/private/var` symlinks) and Windows (handling 8.3 short names), ensuring that path comparisons and click-to-open links work reliably.
+**Action:** Prefer `systemTempDir` over `os.tmpdir()` for all temporary directory operations in `cli-kit`.