Pin third-party GitHub actions

Author: David Cornu

.github/workflows/ci.yml

@@ -46,9 +46,9 @@ jobs:
       DISPLAY: ":99.0"
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3.6.0
       - name: Set up Ruby
-        uses: ruby/setup-ruby@v1
+        uses: ruby/setup-ruby@1a615958ad9d422dd932dc1d5823942ee002799f # v1.227.0
         with:
           ruby-version: ${{ matrix.ruby }}
           # runs 'bundle install' and caches installed gems automatically