Make sure backups read from stdin have a file name

[fd0]

No description provided.

[codecov-io]

Codecov Report

Merging #782 into master will decrease coverage by -0.03%.

@@            Coverage Diff             @@
##           master     #782      +/-   ##
==========================================
- Coverage    54.1%   54.07%   -0.03%     
==========================================
  Files          95       95              
  Lines        7310     7314       +4     
==========================================
  Hits         3955     3955              
- Misses       2775     2778       +3     
- Partials      580      581       +1

Impacted Files	Coverage Δ
src/restic/archiver/archive_reader.go	73.77% <ø> (-2.51%)	❌
src/cmds/restic/cmd_backup.go	28.45% <33.33%> (-0.24%)	❌

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