Set continue-on-error: false for ruby_head and truffleruby workflows

Flip continue-on-error from true to false on the ruby_head and
truffleruby workflows so that a failed CI job actually fails the
suite. continue-on-error: true makes job results "green even when
red", and in practice failures under that mode tend to be ignored —
defeating the purpose of running the job at all.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yahonda

.github/workflows/ruby_head.yml

@@ -8,7 +8,7 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
-    continue-on-error: true
+    continue-on-error: false
     strategy:
       matrix:
         ruby: [

.github/workflows/truffleruby.yml

@@ -8,7 +8,7 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
-    continue-on-error: true
+    continue-on-error: false
     strategy:
       matrix:
         ruby: [