[tutorials][UHI] Fix rendering of tutorials using non-ROOT graphics

[siliataider]

This Pull request:

Some UHI tutorials that render graphics via mplhep cannot produce the canvas png files expected by \macro_image and \macro_output so we get empty frames in the documentation page.

[github-actions]

Test Results

    22 files      22 suites   3d 13h 20m 14s ⏱️
 3 861 tests  3 859 ✅   0 💤 2 ❌
76 285 runs  76 180 ✅ 103 💤 2 ❌

For more details on these failures, see this check.

Results for commit c5e016e.