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.

Fixes #5274

[codecov-io]

Codecov Report

Merging #5427 into master will decrease coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5427      +/-   ##
==========================================
- Coverage   83.64%   83.45%   -0.20%     
==========================================
  Files          37       37              
  Lines        9957     9957              
  Branches     1656     1656              
==========================================
- Hits         8329     8310      -19     
- Misses       1145     1160      +15     
- Partials      483      487       +4     

Impacted Files	Coverage Δ
src/borg/archiver.py	81.01% <100.00%> (ø)
src/borg/platform/__init__.py	72.00% <0.00%> (-12.00%)	⬇️
src/borg/platform/xattr.py	82.53% <0.00%> (-6.35%)	⬇️
src/borg/xattr.py	48.83% <0.00%> (-3.49%)	⬇️
src/borg/platform/base.py	77.58% <0.00%> (-3.45%)	⬇️
src/borg/helpers/misc.py	77.02% <0.00%> (-2.03%)	⬇️
src/borg/archive.py	83.04% <0.00%> (-0.14%)	⬇️

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[ThomasWaldmann]

Thanks!