GitHub actions: No need for setup-nuget

[tautschnig]

As per setup-nuget's documentation, setup-msbuild also takes care of making nuget available. Since we need the latter anyway, just move it earlier in the sequence of steps and remove a dependency (on setup-nuget).

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (e02fd74) 78.94% compared to head (95340fa) 79.01%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7917      +/-   ##
===========================================
+ Coverage    78.94%   79.01%   +0.07%     
===========================================
  Files         1697     1697              
  Lines       195379   195379              
===========================================
+ Hits        154242   154386     +144     
+ Misses       41137    40993     -144     

see 8 files with indirect coverage changes

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