[Performance] Memoize isShopify function

[gonzaloriestra]

Why is this change necessary?

The isShopify function performs a disk I/O check (fileExists) to determine if the dev executable is present. This check is often called multiple times during a single CLI command execution.

How does it address the issue?

This PR introduces memoization for the isShopify function. When called with the default process.env, it stores and returns the same Promise for subsequent calls, avoiding redundant file system operations.

How to test your changes?

CI

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PR created automatically by Jules for task 9584385506099802746 started by @gonzaloriestra

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[github-actions]

Differences in type declarations

We detected differences in the type declarations generated by Typescript for this branch compared to the baseline ('main' branch). Please, review them to ensure they are backward-compatible. Here are some important things to keep in mind:

• Some seemingly private modules might be re-exported through public modules.
• If the branch is behind main you might see odd diffs, rebase main into this branch.

New type declarations

We found no new type declarations in this PR

Existing type declarations

packages/cli-kit/dist/public/node/context/local.d.ts

@@ -33,6 +33,10 @@ export declare function isVerbose(env?: NodeJS.ProcessEnv): boolean;
  * @returns True if the CLI is used in a Shopify environment.
  */
 export declare function isShopify(env?: NodeJS.ProcessEnv): Promise<boolean>;
+/**
+ * Resets the memoized value for the isShopify check.
+ */
+export declare function _resetIsShopify(): void;
 /**
  * This variable is used when running unit tests to indicate that the CLI's business logic
  * is run as a subject of a unit test. We can use this variable to disable output through