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.

We have two variants:

* Install within a Go Module. This is theoretically nice because it'd
  get the same version of the River CLI as the River you're using, but
  has a very large downside currently in that it requires you to add
  the CLI's dependencies to your stack before it works. e.g. Cobra.

* Install outside a Go Module with the equivalent of `@latest` (uses the
  ref of the branch being built in CI). This is currently broken until a
  fix like #35 can come in.

Long term, this job may become too much of a maintenance headache
because Go has some really annoying caching behavior around modules, but
I didn't experience that while writing it, so it might be worthwhile
seeing whether this turns out to be valuable.

.github/workflows/ci.yml

@@ -109,6 +109,79 @@ 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
+
+      #
+      # For Go Module
+      #
+      # This seems like a good idea because it'd get the same version of the CLI
+      # as the Go module uses for its River dependency, but has the considerable
+      # downside of requiring a project to add the CLI's dependencies for it to
+      # work (e.g. Cobra).
+      #
+
+      - name: Make Go Module directory
+        run: mkdir go_module
+
+      - name: Initialize Go module
+        run: go mod init github.com/riverqueue/rivertest
+        working-directory: go_module
+
+      - name: Add River as dependency
+        run: go get github.com/riverqueue/river@${GITHUB_REF#refs/heads/}
+        working-directory: go_module
+
+      # The CLI's dependencies need to be part of the project for this to work
+      # unfortunately.
+      - name: Add CLI dependencies
+        run: go get github.com/spf13/cobra
+        working-directory: go_module
+
+      # To make sure that `go mod tidy` below doesn't blow away the River dep.
+      - name: Add simple Go file
+        run: |
+          cat << EOF > main.go
+          package main
+
+          import (
+              "github.com/spf13/cobra"
+              "github.com/riverqueue/river"
+          )
+
+          func main() {
+              _ = &cobra.Command{}
+              _ = river.NewWorkers()
+          }
+          EOF
+        working-directory: go_module
+
+      - name: Tidy
+        run: go mod tidy
+        working-directory: go_module
+
+      - name: Install in Go module
+        run: go install github.com/riverqueue/river/cmd/river
+        working-directory: go_module
+
+      #
+      # For _not_Go Module
+      #
+
+      - name: Make not Go Module directory
+        run: mkdir not_go_module
+
+      - name: Install in _not_ Go module (with ref)
+        run: go install github.com/riverqueue/river/cmd/river@${GITHUB_REF#refs/heads/}
+        working-directory: not_go_module
+
   golangci:
     name: lint
     runs-on: ubuntu-latest