fix corepack detection for package manager version determination (#11596)

The previous logic was checking for the env var `ENABLE_EXPERIMENTAL_COREPACK` to determine if corepack was being used by a project. However, this value only means that the build system should consider corepack, not that it's actively being used.

We need to check that flag AND the existence of a `packageManager` property in the project's `package.json`.

packages/static-build/src/index.ts

@@ -482,11 +482,15 @@ export const build: BuildV2 = async ({
       spawnOpts.env.CI = 'false';
     }
 
-    const { cliType, lockfileVersion } = await scanParentDirs(entrypointDir);
+    const { cliType, lockfileVersion, packageJson } = await scanParentDirs(
+      entrypointDir,
+      true
+    );
 
     spawnOpts.env = getEnvForPackageManager({
       cliType,
       lockfileVersion,
+      packageJsonPackageManager: packageJson?.packageManager,
       nodeVersion,
       env: spawnOpts.env || {},
     });