Nightly tests: fail a hang after 20 minutes

The nightly run was cancelled at its 120-minute limit every night until
it was disabled. The cause was the Elapsed clock, which kept the test
process alive and is fixed (a909c07): test:all now completes in about
80 seconds. A 20-minute limit leaves a wide margin and turns a future
hang into a failure within the hour rather than two runner-hours.

.github/workflows/test-nightly.yml

@@ -12,7 +12,9 @@ jobs:
         step: ['test:all']
     name: ${{ matrix.step }}
     runs-on: ubuntu-latest
-    timeout-minutes: 120
+    # the full suite runs in about 2 minutes; a hang fails early instead of
+    # holding a runner for hours
+    timeout-minutes: 20
     steps:
     - uses: actions/checkout@v4
     - uses: actions/setup-node@v4