dot is missing from CI

https://github.com/seL4/capdl/actions/runs/26558143019/job/78234438618#step:5:2482

```
dot -Tpng camkes-adder-arm.dot -o camkes-adder-arm.png || true # don't fail the test if graph layout is too hard
/bin/sh: 1: dot: not found
echo placeholder for Makefile >camkes-adder-arm.test-OK
echo placeholder for Makefile >camkes-adder-arm.test-thy-OK
rm example-aarch64.parse
```