A few CI tweaks: env var for database name + better comment

A couple small changes based on what I'm bringing into Python in [1]:

* An env var for database name in addition to URL. Makes it so all
  values are visible in one place right next to each other.

* Improve comment to make it more clear why installing Go is required.

[1] riverqueue/riverqueue-python#14

.github/workflows/ci.yaml

@@ -9,7 +9,8 @@ env:
   BIN_PATH: /home/runner/bin
 
   # A suitable URL for a test database.
-  TEST_DATABASE_URL: postgres://postgres:[email protected]:5432/riverqueue_ruby_test?sslmode=disable
+  TEST_DATABASE_NAME: river_test
+  TEST_DATABASE_URL: postgres://postgres:[email protected]:5432/river_test?sslmode=disable
 
 on:
   - push
@@ -120,16 +121,16 @@ jobs:
           ruby-version: "head"
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically
 
-      # There is a version of Go on Actions' base image, but it's old and can't
-      # read modern `go.mod` annotations correctly.
+      # Needed for River's CLI. There is a version of Go on Actions' base image,
+      # but it's old and can't read modern `go.mod` annotations correctly.
       - name: Install Go
         uses: actions/setup-go@v5
         with:
           go-version: "stable"
           check-latest: true
 
       - name: Create database
-        run: psql --echo-errors --quiet -c '\timing off' -c "CREATE DATABASE riverqueue_ruby_test;" ${ADMIN_DATABASE_URL}
+        run: psql --echo-errors --quiet -c '\timing off' -c "CREATE DATABASE ${TEST_DATABASE_NAME};" ${ADMIN_DATABASE_URL}
 
       - name: Install River CLI
         run: go install github.com/riverqueue/river/cmd/river@latest