Allow --files-cache=size

[ThomasWaldmann]

backport of #5831

[codecov-commenter]

Codecov Report

Merging #5884 (a8ff4ec) into 1.1-maint (71ac031) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           1.1-maint    #5884   +/-   ##
==========================================
  Coverage      79.75%   79.75%           
==========================================
  Files             27       27           
  Lines          10459    10459           
  Branches        1841     1841           
==========================================
  Hits            8342     8342           
+ Misses          1593     1592    -1     
- Partials         524      525    +1     

Impacted Files	Coverage Δ
src/borg/helpers.py	87.84% <100.00%> (-0.08%)	⬇️
src/borg/cache.py	85.46% <0.00%> (+0.12%)	⬆️

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