[CodeHealth] Use base::HeapArray in base/process/environment_internal

Replace std::unique_ptr<T[]> pattern with a base::HeapArray.

Fixed: 326459026
Change-Id: Icd10845c7481134c9773497b0802f08b45338542
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5613531


Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Maria Kazinova <kazinova@google.com>
Cr-Commit-Position: refs/heads/main@{#1312819}