ci: speed up Django tests using cache and running them only once

Use the github cache to cache the poetry virtualenv.
The tests were run both at pull request and pushes - given that we
don't allow merges to main without pull request, lets see if its enough
to run them only on PRs.

.github/workflows/django-tests.yml

@@ -3,7 +3,8 @@
 
 name: Django Test
 
-on: [push, pull_request]
+on: 
+  pull_request:
 
 env:
   DJANGO_SETTINGS_MODULE: tests.settings_ci
@@ -20,16 +21,30 @@ jobs:
       uses: actions/setup-python@v3
       with:
         python-version: ${{ matrix.python-version }}
-    - name: Install dependencies
+
+    - name: Upgrade pip
       run: |
         python -m pip install --upgrade pip
-        pip install poetry
-        poetry config virtualenvs.create false
+
+    - name: Install poetry and setuptools
+      run: |
+        pip install poetry setuptools
+    - name: Configure poetry
+      run: |
+        poetry config virtualenvs.in-project true
+    - name: Cache the virtualenv
+      uses: actions/cache@v2
+      with:
+        path: ./.venv
+        key: ${{ runner.os }}-venv-${{ hashFiles('**/poetry.lock') }}
+    - name: Install dependencies
+      run: |
         poetry lock
         poetry install
+
     - name: Make migrations
-      run: ./manage.py makemigrations
+      run: poetry run ./manage.py makemigrations
     - name: Migrate
-      run: ./manage.py migrate
+      run: poetry run ./manage.py migrate
     - name: Test
-      run: ./manage.py test
+      run: poetry run ./manage.py test