Run Ruff linting with a known-to-work version

This means that CI tests will only fail if the 'known good' version
fails. If the latest version of Ruff fails, perhaps due to new
checks, CI will not fail overall, but the failures from the latest
version will be reported.

.github/workflows/lint.yml

@@ -22,11 +22,18 @@ jobs:
       uses: actions/setup-python@v4
       with:
         python-version: 3
-    - name: Install dependencies
-      run: |
-        python -m pip install --upgrade pip
-        python -m pip install ruff==0.0.259
+    - name: Install pip
+      run: python -m pip install --upgrade pip
+
+    - name: Install known good Ruff
+      run: python -m pip install ruff==0.0.260
+    - name: Lint with known good Ruff
+      run: ruff . --diff --format github
+
+    - name: Install latest Ruff
+      run: python -m pip install --upgrade ruff
     - name: Lint with Ruff
+      continue-on-error: true
       run: ruff . --diff --format github
 
   flake8: