Fix UnivPoly adhoc subtraction

[lgoettgens]

Cherry-picked from #1867.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.13%. Comparing base (193e1b8) to head (278d78d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/UnivPoly.jl	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1881      +/-   ##
==========================================
- Coverage   88.13%   88.13%   -0.01%     
==========================================
  Files         120      120              
  Lines       30278    30282       +4     
==========================================
+ Hits        26687    26689       +2     
- Misses       3591     3593       +2     

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[lgoettgens]

@benlorenz https://github.com/Nemocas/AbstractAlgebra.jl/actions/runs/11575515402/job/32222361228 is another instance where vinberg_2 needs more time than usual. (I restarted the job after about 30 min in this one testfile)