ci: fix 'Verify that the project is built'

The output of that step, if something goes wrong, claims that `dist/` is
not up to date, but the build product is in `lib/`.

Also, `git status -s` shows not only differences in the tracked files,
but also untracked files (which should not exist at that stage). Let's
avoid puzzling contributors when there are untracked files by logging
the output of `git status -s`.

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

Author: dscho

.github/workflows/checkin.yml

@@ -32,7 +32,8 @@ jobs:
       - name: Verify that the project is built
         run: |
           if [[ -n $(git status -s) ]]; then
-            echo "ERROR: generated dist/ differs from the current sources"
+            echo "ERROR: generated lib/ differs from the current sources"
+            git status -s
             git diff
             exit 1
           fi