ci: use sha pinning to mitigate (#245)

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

.github/workflows/linux.yml

@@ -27,8 +27,8 @@ jobs:
         experimental: [false]
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v6
-    - uses: ruby/setup-ruby@v1
+    - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+    - uses: ruby/setup-ruby@4dc28cf14d77b0afa6832d9765ac422dbf0dfedd # v1.298.0
       with:
         ruby-version: ${{ matrix.ruby }}
     - name: unit testing