CI/GHA: cancel outdated CI runs on new PR changes

Avoid letting outdated CI runs continue if a PR receives
new changes. Outside a PR we let them continue running
by tying the concurrency to the commit hash instead.

Also only let one CodeQL or Hacktoberfest job run at a time.

Other CI platforms we use have this build in, but GitHub
unfortunately neither by default nor with a simple option.

This saves CI resources and therefore a little energy.

Approved-by: Daniel Stenberg
Approved-by: Max Dymond
Closes #9533

.github/workflows/linux.yml

@@ -5,8 +5,6 @@
 name: Linux
 
 on:
-  # Trigger the workflow on push or pull requests, but only for the
-  # master branch
   push:
     branches:
     - master
@@ -15,6 +13,10 @@ on:
     branches:
     - master
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
 jobs:
   autotools:
     name: ${{ matrix.build.name }}