.github: update backport to not run on backport PRs

[r1walz]

Description of changes:

Backport workflow is failing on 2.x and other versions due to missing condition on PR title.

CheckList:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7be01d5) 75.06% compared to head (46077ac) 75.03%.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #1113      +/-   ##
============================================
- Coverage     75.06%   75.03%   -0.03%     
+ Complexity     2901     2899       -2     
============================================
  Files           372      372              
  Lines         17160    17160              
  Branches       2476     2476              
============================================
- Hits          12881    12876       -5     
- Misses         2904     2910       +6     
+ Partials       1375     1374       -1     

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