CI masks build failure

In a recent PR build, the CI task crashed in the middle of a build for some reason.

https://github.com/GaloisInc/cryptol/runs/1381112825?check_suite_focus=true#step:8:93

This is annoying, but more critically, the CI job masked this failure and continued on to the testing phase, and only failed because a test designed to exercise the new behavior failed.