File system uses fetch and pages cache only at build phase

[AyronK]

Fix: Limit cache writes to the production build phase.
Fixed #123.

This change ensures that cache writes to the file system using only occur during the build phase.

Changes

• Added a condition to check process.env.NEXT_PHASE === PHASE_PRODUCTION_BUILD before calling CacheHandler.#writePagesRouterPage and CacheHandler.#writeFetch.

Impact

• Prevents file system cache writes at runtime
• Ensures the file system cache isn't unnecessarily modified at runtime (and does not grow larger over time).
• No breaking changes, as this simply restricts file system writes based on the build phase. Preloading cache at instrumentation works as before.