fix: check for nix flake support before trying to load flake

[VVKot]

Which issue does this PR close?

None.

Rationale for this change

#26122 (comment) - users might be running nix that doesn't support flakes.

What changes are included in this PR?

Test for flake support in addition to testing for nix being present.

What is the testing strategy for this PR?

Confirmed that flake is still being loaded when flakes are supported, and not loaded otherwise.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.79%. Comparing base (334f293) to head (e3ed0fc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26188      +/-   ##
==========================================
- Coverage   82.79%   82.79%   -0.01%     
==========================================
  Files        1149     1149              
  Lines      451645   451645              
  Branches   451645   451645              
==========================================
- Hits       373940   373932       -8     
- Misses      54929    54933       +4     
- Partials    22776    22780       +4     

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