CI: don't upload test coverage multiple times

Pick a single Postgres version in the matrix and use it to upload coverage.
Other versions just run tests.

Signed-off-by: Riccardo Binetti <[email protected]>

.github/workflows/backend-test.yaml

@@ -128,6 +128,8 @@ jobs:
     strategy:
       matrix:
         postgres: [13, 14, 15, 16]
+    env:
+      postgres_version_uploading_to_coveralls: 16
     runs-on: ubuntu-22.04
     services:
       postgres:
@@ -174,9 +176,15 @@ jobs:
         mix deps.get --only test
         mix deps.compile
 
-    - name: Test
+    # Only send upload coverage from a single test in the matrix
+    - name: Test and upload coverage
+      if: matrix.postgres == env.postgres_version_uploading_to_coveralls
       run: mix coveralls.github -o coverage_results --warnings-as-errors
 
+    - name: Test
+      if: matrix.postgres != env.postgres_version_uploading_to_coveralls
+      run: mix test
+
   build-docker-image:
     name: Build Docker image
     runs-on: ubuntu-latest