Ref actions by commit SHA in ci.yml

It's important to make sure the SHA's are from the original repositories and not forks.

For reference:

https://github.com/actions/checkout/releases/tag/v4.0.0
3df4ab11eb

https://github.com/actions/cache/releases/tag/v3.3.2
704facf57e

Signed-off-by: Gabriela Gutierrez <gabigutierrez@google.com>

.github/workflows/ci.yml

@@ -5,9 +5,9 @@ jobs:
     runs-on: ubuntu-latest
     name: Build and test
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac #v4.0.0
 
-      - uses: actions/cache@v2
+      - uses: actions/cache@704facf57e6136b1bc63b828d79edcd491f0ee84 #v3.3.2
         with:
           path: '/home/runner/work/codemirror/codemirror5/node_modules'
           key: ${{ runner.os }}-modules