Change mode when creating a directory

[0405ysj]

Recently I faced at the permission related issue when doing POST /cvds after GET /cvds.

And found the root cause _cutf-operator trying to change mode of /tmp/cvd directory, which is owned by _cvd-executor.

I can resolve this issue with this PR, but I still have doubts on:

• Why can't mkdir function set the mode?
• Why doesn't the group of _cvd-executor set as cvdnetwork?
• Why are the users different between GET /cvds and POST /cvds?