ci: try to fix r deps

.github/workflows/check.yml

@@ -31,6 +31,22 @@ jobs:
         sudo apt-get install -y xvfb libxkbcommon-x11-0 libxcb-icccm4 libxcb-image0 libxcb-keysyms1 libxcb-randr0 libxcb-render-util0 libxcb-shape0 libxcb-xinerama0 libxcb-xinput0 libxcb-xfixes0
         # start xvfb in the background
         sudo /usr/bin/Xvfb $DISPLAY -screen 0 1280x1024x24 &
+    - name: install R
+      if: matrix.extras == 'all'
+      uses: r-lib/actions/setup-r@v2
+      with:
+        r-version: '3.6.3'
+    - name: install lme4
+      if: matrix.extras == 'all'
+      uses: r-lib/actions/setup-r-dependencies@v2
+      with:
+        extra-packages: any::lme4
+    - name: Downgrade rpy2 (Windows)
+      if: runner.os == 'Windows' && matrix.extras == 'all'
+      run: |
+        # Force installation of rpy2 3.5.12
+        # https://github.com/rpy2/rpy2/issues/1044
+        python -m pip install rpy2==3.5.12
     - name: Install Python dependencies
       run: |
         # prerequisites