ci: Update Dotty workflow to install .NET 9.0

[tippmar-nr]

I forgot that you have to install .NET 9.0 on ubuntu-latest runners when I updated Dotty a week or two ago.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.28%. Comparing base (995ffd8) to head (d3e75fd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3098   +/-   ##
=======================================
  Coverage   82.28%   82.28%           
=======================================
  Files         477      477           
  Lines       30636    30636           
  Branches     3424     3424           
=======================================
  Hits        25208    25208           
  Misses       4626     4626           
  Partials      802      802           

Flag	Coverage Δ
Profiler	72.51% <ø> (ø)

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