Ability to set --cache-dir and --with-cache on check

Apparently restic always creates a new directory in /tmp (by default) when running a "check" command. I don't have enough space on my /tmp and have to redirect.

https://forum.restic.net/t/restic-0-9-0-check-fills-up-my-tmp-partition/679

Maybe I'm missing the obvious...

And reading more, you can set:

TMPDIR=~/.restic-tmp

Which may take care of it... sorry, missed that, will test more.

Thanks, great script!