export-tar: set tar format to GNU_FORMAT explicitly

[enkore]

Python 3.8 changed the default format to PAX, but GNU format is
documented for Borg.

Backport of #5427 to 1.1 to fix #5274 in next maintenance release.

[codecov-io]

Codecov Report

Merging #5428 into 1.1-maint will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           1.1-maint    #5428      +/-   ##
=============================================
- Coverage      80.33%   80.30%   -0.03%     
=============================================
  Files             27       27              
  Lines          10358    10358              
  Branches        1827     1827              
=============================================
- Hits            8321     8318       -3     
- Misses          1525     1527       +2     
- Partials         512      513       +1     

Impacted Files	Coverage Δ
src/borg/archiver.py	82.04% <100.00%> (ø)
src/borg/repository.py	85.23% <0.00%> (-0.20%)	⬇️
src/borg/archive.py	82.98% <0.00%> (-0.15%)	⬇️
src/borg/cache.py	86.03% <0.00%> (+0.13%)	⬆️

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