Add module support for --stdin-from-command flag, which was added to
restic in https://github.com/restic/restic/pull/4410. I also made a few
very small changes here and there in the nix code to make it look a
little neater in my opinion.
I could potentially add support for the --stdin flag too, but this would
require prepending the restic command with an external command and a
pipe, which seems a bit messy - and the restic documentation says to
prefer --stdin-from-command over --stdin anyway.
I could add an option for --stdin-filename, but I feel that this would
be better for users to do in extraBackupArgs.
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