Fix workflow marked as failed despite "continue-on-error" enabled

When "continue-on-error" is set at the job's level, the entire workflow
wont be considered as "failed" per se. However, the icon associated with
a commit will still be a red cross mark "X". This is confusing, this
could incorrectly suggest that the build has failed.

By putting "continue-on-error" at the step's level, the job and
therefore the full workflow will be marked as succeeded. This is
preferable, even if it makes problems less noticeable.

.github/workflows/tests.yml

@@ -22,7 +22,6 @@ jobs:
         - '3.11'
         - '3.12'
         - '3.13'
-        - 3.14-dev
         - pypy-3.10
         include:
         - os: ubuntu-20.04
@@ -33,8 +32,10 @@ jobs:
           python-version: '3.12'
         - os: macos-13
           python-version: '3.12'
+        - os: ubuntu-22.04
+          python-version: 3.14-dev
+          allow-failure: true
     runs-on: ${{ matrix.os }}
-    continue-on-error: ${{ endsWith(matrix.python-version, '-dev') }}
     steps:
     - name: Checkout repository
       uses: actions/checkout@v4
@@ -52,10 +53,12 @@ jobs:
     - name: Run tests
       run: |
         tox -e tests
+      continue-on-error: ${{ matrix.allow-failure }}
     - name: Upload coverage to Codecov
       uses: codecov/codecov-action@v4
       with:
         token: ${{ secrets.CODECOV_TOKEN }}
         flags: ${{ matrix.os }}_${{ matrix.python-version }}
         verbose: true
         fail_ci_if_error: true
+      continue-on-error: ${{ matrix.allow-failure }}