feat(github): auto cancel previous continuous-integration.yml run when a PR is updated

.github/workflows/auto-cancel.yml

@@ -0,0 +1,14 @@
+# https://github.com/styfle/cancel-workflow-action#user-content-advanced-canceling-other-workflows
+name: Auto-cancel-outdated PR
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+jobs:
+  cancel:
+    name: Auto-cancel-outdated PR
+    runs-on: ubuntu-latest
+    steps:
+      - uses: styfle/cancel-workflow-action@0.10.0
+        with:
+          workflow_id: all
+          access_token: ${{ github.token }}

.github/workflows/continuous-integration.yml

@@ -56,10 +56,10 @@ jobs:
     strategy:
       matrix:
         compiler: [gcc7, latest]
-    # We want to always run the tests, but after the docker image build job.
+    # We want to always run the tests unless cancelled, but after the docker image build job.
     # If there is no new build then we use the old image, else we use the new image.
     # The latest compiler job isn't using the built image, so it suffers the dependance of the gcc7 job.
-    if: ${{ always() }}
+    if: ${{ !cancelled() }}
     needs: [BuildAndPushDockerImages]
     steps:
       # Free 4Gb of workspace
@@ -307,9 +307,9 @@ jobs:
 
   BuildAndTestDF:
     runs-on: ubuntu-latest
-    # We want to always run the tests, but after the docker image build job.
+    # We want to always run the tests unless cancelled, but after the docker image build job.
     # If there is no new build then we use the old image, else we use the new image.
-    if: ${{ always() }}
+    if: ${{ !cancelled() }}
     needs: [BuildAndPushDockerImages]
     steps:
       # A SSH private key is required as some dependencies are from private repos