Bugfix: Make cache crash-resistant and usable by multiple concurrent processes

The restic cache directory ($RESTIC_CACHE_DIR) could end up in a broken state
in the event of restic (or the OS) crashing. This is now less likely to occur
as files are downloaded to a temporary location before being moved in place.

This also allows multiple concurrent restic processes to operate on a single
repository without conflicts. Previously, concurrent operations could cause
segfaults because the processes saw each other's partially downloaded files.

https://github.com/restic/restic/issues/2345
https://github.com/restic/restic/pull/2838