ci(backend): don't run backend ci if files haven't changed (#6371)

This is a second attempt, see
https://github.com/PostHog/posthog/pull/6338 and the revert,
https://github.com/PostHog/posthog/pull/6356 which has some more detail.

Essentially we can't just use github action path filtering because we
have specified some jobs as required. So this will still keep the jobs
around, but they just won't do anything.

.github/workflows/ci-backend.yml

@@ -13,7 +13,39 @@ env:
     TEST: 1
 
 jobs:
+    # JOB to decide if we should run backend ci. See
+    # https://github.com/dorny/paths-filter#conditional-execution for more
+    # details
+    changes:
+        runs-on: ubuntu-latest
+        # Set job outputs to values from filter step
+        outputs:
+            backend: ${{ steps.filter.outputs.backend }}
+        steps:
+            # For pull requests it's not necessary to checkout the code
+            - uses: dorny/paths-filter@v2
+              id: filter
+              with:
+                  filters: |
+                      backend:
+                        # Avoid running backend tests for irrelevant changes
+                        # NOTE: we are at risk of missing a dependency here. We could make
+                        # the dependencies more clear if we separated the backend/frontend
+                        # code completely
+                        - 'ee/**/*'
+                        - 'posthog/**/*'
+                        - requirements.txt
+                        - requirements-dev.txt
+                        - mypy.ini
+                        - pytest.ini
+                        # Make sure we run if someone is explicitly change the workflow
+                        - .github/workflows/ci-backend.yml
+
     backend-code-quality:
+        needs: changes
+        # Make sure we only run on backend changes
+        if: ${{ needs.changes.outputs.backend == 'true' }}
+
         name: Code quality checks
         runs-on: ubuntu-latest
 
@@ -69,6 +101,9 @@ jobs:
                   mypy .
 
     django:
+        needs: changes
+        if: ${{ needs.changes.outputs.backend == 'true' }}
+
         name: Django tests – Py ${{ matrix.python-version }}
         runs-on: ubuntu-latest
 
@@ -134,6 +169,9 @@ jobs:
                   verbose: true
 
     saml:
+        needs: changes
+        if: ${{ needs.changes.outputs.backend == 'true' }}
+
         name: Django tests – with SAML
         runs-on: ubuntu-latest
         steps:
@@ -187,6 +225,9 @@ jobs:
                   pytest posthog -m "ee"
 
     cloud:
+        needs: changes
+        if: ${{ needs.changes.outputs.backend == 'true' }}
+
         name: Django tests – Cloud
         runs-on: ubuntu-latest
         steps:
@@ -286,6 +327,9 @@ jobs:
                   pytest posthog -m "ee and not skip_on_multitenancy"
 
     foss:
+        needs: changes
+        if: ${{ needs.changes.outputs.backend == 'true' }}
+
         name: Django tests – FOSS
         runs-on: ubuntu-latest