fix(tvix/castore/directory/sled): use spawn_blocking r/8101

This does IO, which might take a longer amount of time than what we want
to be blocking the normal executor.

Use spawn_blocking instead. I didn't add it for the constructors, as we
only call these once.

Change-Id: I96231fcff8d10abe90cafde25a099a2db6ea9414
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11617
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

0 files changed, 0 insertions, 0 deletions