CI currently failing on missing PNG output for dot

The CI on the master branch is failing on 2 tests from beat/beat.cmdline (involving the use of graphviz via a draw command).

The relevant output is the following (from, e.g., https://gitlab.idiap.ch/beat/beat.web/-/jobs/167089):

Warning: Could not load "/scratch/builds/iJyPBhV4/0/beat/beat.web/miniconda/envs/beat-web/lib/graphviz/libgvplugin_pango.so.6" - file not found
Warning: Could not load "/scratch/builds/iJyPBhV4/0/beat/beat.web/miniconda/envs/beat-web/lib/graphviz/libgvplugin_pango.so.6" - file not found
Format: "png" not recognized. Use one of: canon cmap cmapx cmapx_np dot dot_json eps fig gv imap imap_np ismap json json0 mp pdf pic plain plain-ext png pov ps ps2 svg svgz tk vml vmlz xdot xdot1.2 xdot1.4 xdot_json
...
subprocess.CalledProcessError: Command '['dot', '-Tpng', '-O', '/tmp/beat.core.testw6ox_zkl.tmpdir/experiments/user/user/double_triangle/1/double_triangle']' returned non-zero exit status 1.

So, the dot command is failing on the beat.web CI, which uses the raw shell in the builder.

The test of beat/beat.cmdline passes with flying colors (https://gitlab.idiap.ch/beat/beat.web/-/jobs/167089), but all drawing tests are skipped in this case!

On the mac this seems to work flawlessly, but not on the shell-based CI.