ci: a runner that refuses unshare passes build-and-test with only a warning #107

In .github/workflows/ci.yml (lines 102-104 at af0abbe), the else branch taken when the runner refuses unshare prints a ::warning and runs make sabotage without the LV2 guard. So the comment at line 81, "a run that skipped them fails here", holds only for the then-branch: a run that skipped the JACK and LV2 scenarios still goes green.

The fix is to make the else-branch exit 1, or to require the two ok <scenario> lines in both branches. Found while reviewing #104.