Add CI check to make sure River CLI is installable

Pertaining to the problem described in #35, seems like a good idea to
have a CI check make sure that nothing we've done with Go modules has
made the CLI not installable.

.github/workflows/ci.yml

@@ -109,6 +109,28 @@ jobs:
       - name: river migrate-down
         run: ./river migrate-down --database-url $DATABASE_URL --max-steps 100
 
+  cli_installable:
+    runs-on: ubuntu-latest
+    timeout-minutes: 3
+
+    steps:
+      - uses: actions/setup-go@v4
+        with:
+          go-version: "stable"
+          check-latest: true
+
+      - run: mkdir go_module
+
+      - name: Install in Go module
+        run: go install github.com/riverqueue/river/cmd/river
+        working-directory: go_module
+
+      - run: mkdir not_go_module
+
+      - name: Install in _not_ Go module (with ref)
+        run: go install github.com/riverqueue/river/cmd/river@${GITHUB_REF#refs/heads/}
+        working-directory: not_go_module
+
   golangci:
     name: lint
     runs-on: ubuntu-latest