Install mongosh in CI workflow for index creation

The previous commit added an index creation step that requires mongosh,
but mongosh is not installed on the Ubuntu runner by default.
This adds a step to download and install mongosh-2.3.8-linux-x64.

Author: cbullinger

.github/workflows/run-express-tests.yml

@@ -41,6 +41,12 @@ jobs:
       - name: Add sample data to database
         run: mongorestore --archive=sampledata.archive --port=27017
 
+      - name: Install MongoDB Shell (mongosh)
+        run: |
+          curl https://downloads.mongodb.com/compass/mongosh-2.3.8-linux-x64.tgz -o mongosh.tgz
+          tar -xzf mongosh.tgz
+          sudo cp mongosh-2.3.8-linux-x64/bin/* /usr/local/bin/
+
       - name: Create indexes for aggregation performance
         run: |
           mongosh "mongodb://localhost:27017/sample_mflix?directConnection=true" --eval "db.comments.createIndex({ movie_id: 1 })"