Run Enzyme tests only on CUDA CI machine

[pxl-th]

Since Enzyme explicitly adds CUDA to its environment, run its tests only on CUDA CI machines to prevent crashing other jobs.

FYI @wsmoses

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.32%. Comparing base (d1ff714) to head (3dfe290).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2478      +/-   ##
==========================================
+ Coverage   36.38%   41.32%   +4.94%     
==========================================
  Files          37       37              
  Lines        2119     2110       -9     
==========================================
+ Hits          771      872     +101     
+ Misses       1348     1238     -110     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[wsmoses]

@pxl-th hm this is a bit weird, since Enzyme CPU tests would be equally good to check, no?

[pxl-th]

Then it is probably better to run Enzyme CPU tests when CPU tests are enabled and Enzyme CUDA tests when CUDA tests are enabled.
We were getting this crashes on other backends, because Enzyme was trying to run CUDA tests on them:
https://buildkite.com/julialang/flux-dot-jl/builds/4820#01916a11-6182-4203-b661-ad9c97b96c86

[ToucheSir]

Buildkite should still be running Enzyme CPU tests as well.