fix: run the installer under python 3.8 in ci

.github/workflows/validate.yml

@@ -122,9 +122,14 @@ jobs:
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7
 
+      # install.py stays on Python 3.8: the suite runs it end to end under a
+      # real 3.8 interpreter, which import-only and grammar checks miss. The
+      # last version listed is the default python.
       - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97  # v7
         with:
-          python-version: "3.12"
+          python-version: |
+            3.8
+            3.12
 
       - name: Install dependencies
         run: pip install pyyaml "jsonschema[format-nongpl]==4.26.0"