Revert "cbuildbot_launch: Update the location of the source cache"

This reverts commit d9e62e0a393bb104918a1d4b61fca6c203d3f227.

Reason for revert: Due to the way preload works, you cannot link across devices thus causing this to copy the entire disk to named cache.  The duration is not acceptable thus backing this out until I can refactor how the initial sync works.

Original change's description:
> cbuildbot_launch: Update the location of the source cache
>
> Update the location of the source cache checkouts as we move from
> nightly image preloads to source snapshots.
>
> BUG=b:197678613
> TEST='cros tryjob'
>
> Change-Id: I3a9144e2133f8c423cb4a3b5d760fdcafa69d460
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3118813
> Reviewed-by: Jack Neus <jackneus@google.com>
> Reviewed-by: Jared Loucks <jaredloucks@google.com>
> Reviewed-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
> Commit-Queue: Mike Nichols <mikenichols@chromium.org>
> Tested-by: Mike Nichols <mikenichols@chromium.org>

Bug: b:197678613
Change-Id: I4aa6cc35006ff24679768e488c7220998d66e6df
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3119598
Commit-Queue: Mike Nichols <mikenichols@chromium.org>
Tested-by: Mike Nichols <mikenichols@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
diff --git a/scripts/cbuildbot_launch.py b/scripts/cbuildbot_launch.py
index 59796d1..7693e36 100644
--- a/scripts/cbuildbot_launch.py
+++ b/scripts/cbuildbot_launch.py
@@ -347,7 +347,7 @@
   cbuildbot_alerts.PrintBuildbotStepText('Branch: %s' % repo.branch)
   logging.info('Bootstrap script starting initial sync on branch: %s',
                repo.branch)
-  repo.PreLoad('/snapshot_mounts/chromiumos')
+  repo.PreLoad('/preload/chromeos')
   repo.Sync(detach=True,
             downgrade_repo=_ShouldDowngradeRepo(options))