[Performance] Memoize isShopify environment check

[gonzaloriestra]

Description

This PR implements memoization for the isShopify function in @shopify/cli-kit. This function determines if the CLI is running in a Shopify internal development environment by checking environment variables and performing a file existence check for the /opt/dev/bin/dev executable.

Since isShopify is called multiple times during a CLI command's lifecycle (e.g., in analytics reporting), caching the result promise prevents redundant disk I/O. The memoization is restricted to calls using the default process.env and is bypassed during unit tests to ensure test isolation.

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Checklist

• I've added a changeset (if applicable)
• I've added tests for my changes.
• I've extended the documentation (if applicable)
• I've made sure that my changes are accessible to all users.

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PR created automatically by Jules for task 1733951532173800447 started by @gonzaloriestra

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