CI workflow: install dependencies for the test jobs too

shelltestrunner was already installed for the coverage target, but not
for main test ones (for the same reason that release 1.5 was broken,
in the sense that it was not needed…).

.github/workflows/ci.yml

@@ -33,6 +33,9 @@ jobs:
       - name: Build the code
         run: make
 
+      - name: Install dependencies
+        run: sudo apt-get install -yy shelltestrunner
+
       - name: Test creating the release archive
         run: make distcheck