GHA: Cancel in-progress jobs if pushing to PR

See https://docs.github.com/en/actions/using-jobs/using-concurrency
Should reduce the resource usage a bit and may speed up things.

Author: amotl

.github/workflows/docs.yml

@@ -9,6 +9,10 @@ on:
   schedule:
     - cron: '0 7 * * *'
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   documentation:
 

.github/workflows/tests.yml

@@ -7,6 +7,9 @@ on:
     branches: [ master ]
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   test: