ci: test against the three main package managers

.github/workflows/ci.yml

@@ -38,6 +38,7 @@ jobs:
       # how many fail)
       fail-fast: false
       matrix:
+        js_package_manager: [npm, pnpm, yarn]
         variant:
           - name: defaults
             config_path: "ackama_rails_template.config.yml"
@@ -116,6 +117,9 @@ jobs:
           cache: "yarn"
           node-version-file: ".node-version"
 
+      - name: install package manager
+        run: npm i -g ${{ matrix.js_package_manager }}
+
       # We don't cache gems or JS packages because we are actually testing how
       # installation and setup works in this project so, while caching would
       # make CI faster, it might hide problems.
@@ -149,4 +153,5 @@ jobs:
           PGUSER: postgres
           PGPASSWORD: postgres
           PGHOST: localhost
+          PACKAGE_JSON_FALLBACK_MANAGER: ${{ matrix.js_package_manager }}
         run: ./ci/bin/build-and-test