borgbackup: make logs a bit cleaner if local archive is used

[szaimen]

Follow-up to #4804

Prevent logs like the following when used with local archive:

cat: can't open '.pub': No such file or directory
Your public ssh key for borgbackup is: 

[timdiels]

Oops, LGTM

[timdiels]

Nit: I would merge both ifs

• if remote
  • if key does not exist, create it
  • if key exists, cat it

[szaimen]

Nit: I would merge both ifs

• if remote

  • if key does not exist, create it
  • if key exists, cat it

I see... Do you want to create a PR? :)

[timdiels]

No no, either is fine really