fix: xattr xdg backup exclusion should be on 'false'

[Flachz]

Description

According to the freedesktop proposal the xattrs attribute user.xdg.robots.backup should be set to false to exclude files from backups.

The current implementation has it backwards, as files are excluded if user.xdg.robots.backup is set to true.

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.75%. Comparing base (fea52ef) to head (e19232b).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archive.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9666      +/-   ##
==========================================
+ Coverage   83.62%   83.75%   +0.12%     
==========================================
  Files          90       90              
  Lines       15539    15539              
  Branches     2337     2337              
==========================================
+ Hits        12995    13015      +20     
+ Misses       1806     1789      -17     
+ Partials      738      735       -3     

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[ThomasWaldmann]

Ah, crap, clicked too fast...

#9667