ci: verify that the action.yml files are in sync

In #218, I added a
convenient way to launch this Action in detached mode:
`mxschmitt/action-tmate/detached@v3`.

The way this is implemented is a copy/edited version of `action.yml` in
the `detached/` subdirectory.

This runs the danger of inadvertent divergences, as happened in
#221 (which I caught in
time and the contributor gracefully addressed).

Let's add automation not only to update the file easily but also to
cause a failure in the PR build with a helpful message suggesting how to
fix the problem.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/checkin.yml

@@ -13,6 +13,14 @@ jobs:
         uses: actions/setup-node@v3
         with:
           node-version: 20
+      - name: Verify that action.yml files are in sync
+        run: |
+          npm run update-detached-action.yml &&
+          if ! git diff --exit-code \*action.yml
+          then
+            echo ''::error::action.yml files are not in sync, maybe run `npm run update-detached-action.yml`?'
+            exit 1
+          fi
       - name: Install dependencies
         run: npm ci
       - name: Run tests

package.json

@@ -7,6 +7,7 @@
   "scripts": {
     "start": "node src/index.js",
     "build": "ncc build src/main.js -o lib",
+    "update-detached-action.yml": "sed '/^runs:$/{N;N;N;s/lib\\//..\\/&/g;};/^ detached:/{N;N;N;s/\\(default: .\\)false/\\1true/g;}' action.yml > action-detached.yml",
     "test": "GITHUB_EVENT_PATH= jest"
   },
   "repository": {