tests: stop running workflows on push

Also remove unnecessary strategy block from jobs.

.github/workflows/ci.yml

@@ -10,12 +10,14 @@ on:
     branches:
       - master
 
+concurrency:
+  group: build-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   unit-tests-master:
     name: unit-tests
     runs-on: ubuntu-20.04
-    strategy:
-      fail-fast: true
     env:
       DOKKU_VERSION: master
 
@@ -58,8 +60,6 @@ jobs:
   unit-tests-0_19_0:
     name: unit-tests-0.19.0
     runs-on: ubuntu-20.04
-    strategy:
-      fail-fast: true
     env:
       DOKKU_TAG: v0.19.0