update github actions to commits (#24)

Author: rxbchen

.github/dependabot.yaml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: weekly

.github/workflows/ci.yml

@@ -17,8 +17,8 @@ jobs:
           '3.2',
         ]
     steps:
-    - uses: actions/checkout@v2
-    - uses: ruby/setup-ruby@v1
+    - uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5 # v2.7.0
+    - uses: ruby/setup-ruby@dffc446db9ba5a0c4446edb5bca1c5c473a806c5 # v1.235.0
       with:
         ruby-version: ${{ matrix.ruby }}
         bundler-cache: true
@@ -27,8 +27,8 @@ jobs:
   valgrind:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: ruby/setup-ruby@v1
+      - uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5 # v2.7.0
+      - uses: ruby/setup-ruby@dffc446db9ba5a0c4446edb5bca1c5c473a806c5 # v1.235.0
         with:
           bundler-cache: true
       - run: sudo apt-get install -y valgrind