💚(ci) download mails templates when testing back

build-mails job builds mails Django templates but was not persisting its
output. This steps was present in Joanie CI. It might have been removed,
when converting Circle CI worflows to Github Actions.

Artifacts are passed between build-mails and test-back jobs. test-back
job has now a dependency to  build-mails.

.github/workflows/people.yml

@@ -174,6 +174,11 @@ jobs:
         run: yarn install --frozen-lockfile
       - name: Build mails
         run: yarn build
+      - name: Persist mails' templates
+        uses: actions/upload-artifact@v4
+        with:
+            name: mails-templates
+            path: src/backend/core/templates/mail
 
   lint-back:
     runs-on: ubuntu-latest
@@ -198,6 +203,7 @@ jobs:
 
   test-back:
     runs-on: ubuntu-latest
+    needs: build-mails
     defaults:
       run:
         working-directory: src/backend
@@ -232,6 +238,11 @@ jobs:
         run: |
           sudo mkdir -p /data/media && \
           sudo mkdir -p /data/static
+      - name: Download mails' templates
+        uses: actions/download-artifact@v4
+        with:
+          name: mails-templates
+          path: src/backend/core/templates/mail
       - name: Install Python
         uses: actions/setup-python@v3
         with: