ci: Pin Poetry to 1.8.5

Poetry 2.0.0 removes the `export` plugin the license check relies on. Additionally it requires Python 3.9 or later and I still want to support older Python versions.

.github/workflows/precommit.yml

@@ -12,7 +12,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Install poetry
-        run: pipx install poetry
+        run: pipx install poetry==1.8.5
       - uses: actions/setup-python@v4
         with:
           python-version: "3.8"