CI downstream testing of mirgecom is failing

[MTCam]

https://github.com/inducer/grudge/actions/runs/3464073343/jobs/5785152287#step:3:1041
This is due to changes in mirgecom/examples/run_examples.sh where an unbound var is checked.

Using this interface to run the tests will fix the issue:
#292

Or... changing the scripts to always have a default value for empty vars will fix it, too. This change set is being tested in mirgecom:
illinois-ceesd/mirgecom#799