Support read-only repositories by using --no-lock.

[jmigual]

I've added the --no-lock flag and everything seemed to work.

Thins I've tested:

• Opening an S3 repository
• Browsing files from a backup of the repository
• Downloading files.
• Restoring a folder

All of this seemed to work fine.

Related issue #144

[emuell]

I think that deserves a comment. Else this looks good to me.

Maybe something like:

// all expected restic operations are read-only right now, so always avoid locks
args.push(Cow::Borrowed(OsStr::new("--no-lock")));

?

[jmigual]

Good point! I've updated it. Thanks for the fast response and for your amazing software! 😊

[emuell]

Thanks for the contribution!