pgo: Use atime to avoid changing profile mtime

The script was updating the modification time (mtime) of PGO profiles on
every build. This can interfere with siso, which relies on mtime to
track file changes.

This CL updates the script to modify the access time (atime) instead.
Profile cleanup now also uses atime, preserving the original mtime for
build tooling while still allowing unused profiles to be cleaned up.

R=jeffyoon@google.com

Bug: 434010397, 364987714, 433993087
Fixed: 434010397
Change-Id: I7db7729c4ccb14cb53cb6a41bbe707c16f9b2245
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6785187
Commit-Queue: Peter Wen <wnwen@chromium.org>
Commit-Queue: Jeff Yoon <jeffyoon@google.com>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Reviewed-by: Jeff Yoon <jeffyoon@google.com>
Cr-Commit-Position: refs/heads/main@{#1492152}