GitHub actions: replace deprecated set-output

[tautschnig]

Use environment files as per
https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/.

• 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

Base: 78.00% // Head: 78.00% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (abf93f5) compared to base (062bc0c).
Patch coverage: 37.83% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7256   +/-   ##
========================================
  Coverage    78.00%   78.00%           
========================================
  Files         1621     1621           
  Lines       187279   187273    -6     
========================================
+ Hits        146088   146089    +1     
+ Misses       41191    41184    -7     

Impacted Files	Coverage Δ
src/cprover/axioms.cpp	92.11% <ø> (+0.54%)	⬆️
src/cprover/counterexample_found.cpp	93.33% <ø> (+11.19%)	⬆️
src/cprover/solver_progress.cpp	60.00% <0.00%> (-20.00%)	⬇️
src/cprover/state_encoding.cpp	80.57% <ø> (+0.22%)	⬆️
src/cprover/solver.cpp	81.53% <40.90%> (-1.62%)	⬇️
src/cprover/propagate.cpp	73.52% <44.44%> (-1.48%)	⬇️
src/solvers/smt2/smt2_format.cpp	85.39% <100.00%> (ø)
src/cprover/solver_types.h	88.88% <0.00%> (-0.59%)	⬇️
src/solvers/smt2/smt2_conv.cpp	65.47% <0.00%> (+0.06%)	⬆️
... and 3 more

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