backup: Fix deadlock

[fd0]

When the archiver is faster than the scanner, restic deadlocks. This
commit adds a finished channel to the struct in ui/backup.go so that
scanner results are ignored when the archiver is already finished.

Closes #1834

[codecov-io]

Codecov Report

Merging #1835 into master will decrease coverage by 0.18%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1835      +/-   ##
==========================================
- Coverage   51.14%   50.95%   -0.19%     
==========================================
  Files         167      167              
  Lines       12915    12921       +6     
==========================================
- Hits         6605     6584      -21     
- Misses       5311     5340      +29     
+ Partials      999      997       -2

Impacted Files	Coverage Δ
internal/archiver/scanner.go	67.85% <0%> (-5.22%)	⬇️
internal/ui/backup.go	0% <0%> (ø)	⬆️
internal/checker/checker.go	63.82% <0%> (-3.83%)	⬇️
internal/backend/rclone/backend.go	64.32% <0%> (-1.76%)	⬇️
cmd/restic/global.go	27.37% <0%> (-0.87%)	⬇️
internal/archiver/file_saver.go	85.96% <0%> (+1.75%)	⬆️

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