Fix copying of symlinks.

Use a different flag when copying the package contents to preserve
symlinks instead of resolving them. This will save about 7Gb in the
package size.

Bug: 1340010
No-Presubmit: true
Change-Id: I8478c15dee94dfb6e247ed0bfbc1a50d81119a5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3760195
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Mike Pinkerton <pinkerton@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1023755}