CI: Set continue-on-error false for Ruby Head

Make a job failed when a test is failed.

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

Author: ashie

.github/workflows/test-ruby-head.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   test:
     runs-on: ${{ matrix.os }}
-    continue-on-error: true
+    continue-on-error: false
     strategy:
       fail-fast: false
       matrix: