Generate toolchain name inside condition

[LucasSte]

Problem

Rustup may not be available in nix-environments, so we cannot rely on it for generating the toolchain name. See the comment in #6981 (comment).

Summary of Changes

Only generate the toolchain name if we are not overriding rustup.

[joncinque]

Thanks for taking care of this!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.2%. Comparing base (b4ba976) to head (434493d).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #7024     +/-   ##
=========================================
- Coverage    83.2%    83.2%   -0.1%     
=========================================
  Files         853      853             
  Lines      375129   375129             
=========================================
- Hits       312239   312214     -25     
- Misses      62890    62915     +25     

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