fix(@next/env): respect loadEnvConfig cache per directory

[raashish1601]

Fixes #92040.

Summary

• reuse the cached @next/env result only when loadEnvConfig is called for the same resolved directory
• reset the previous loaded-file list when switching directories so reload bookkeeping does not leak across roots
• add a regression that loads two app roots back to back without orceReload

Testing

• git diff --check
• manual
  ode --experimental-strip-types harness against packages/next-env/index.ts covering process-env identity and two-directory reload behavior

[nextjs-bot]

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[nextjs-bot]

Allow CI Workflow Run

• approve CI run for commit: 7332a4d

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer