fix(formats): avoid CSV autodetection for generated CSV files from Excel

[nijel]

Proposed changes

Depends on translate/translate#5267

Fixes #11872

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.60%. Comparing base (6aee586) to head (a51ff42).
Report is 3978 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11885      +/-   ##
==========================================
- Coverage   90.82%   90.60%   -0.22%     
==========================================
  Files         554      576      +22     
  Lines       57306    58777    +1471     
  Branches     9122     9383     +261     
==========================================
+ Hits        52046    53254    +1208     
- Misses       3640     3829     +189     
- Partials     1620     1694      +74     

Files with missing lines	Coverage Δ
weblate/formats/external.py	88.37% <100.00%> (-2.81%)	⬇️
weblate/formats/ttkit.py	90.84% <100.00%> (-0.57%)	⬇️

... and 326 files with indirect coverage changes