cros-pvs: save version file instead of manifest

Update the prepper to save the version file in the docker container
instead of the manifest file because the version file has the cipd ref

BUG=b:299296496
TEST=built pvs and ran shop unpack and verified the version files were
in the container

Change-Id: I844971e12b7b8b4ab8f3aa969191b9c8f9be7c01
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4847977
Tested-by: Jack Gelinas <jackgelinas@google.com>
Auto-Submit: Jack Gelinas <jackgelinas@google.com>
Reviewed-by: Brett Brotherton <bbrotherton@google.com>
Commit-Queue: Jack Gelinas <jackgelinas@google.com>
1 file changed