ci: Disable fail-fast in GitHub Tests workflow

Don't cancel all the jobs if one distro config fails. The jobs are
mostly independent, so we do want to let the others continue in case
the failure is isolated to that particular distro configuration.

.github/workflows/tests.yml

@@ -30,6 +30,9 @@ jobs:
     #
     # configure-options: Options to pass to `configure`.
     strategy:
+      # Let other configurations continue if one fails.
+      fail-fast: false
+
       matrix:
         include:
           - name: Debian Buster (10) with sign-ed25519