ci: Cancel running workflows if retriggered

For rationale, see
<https://turso.tech/blog/simple-trick-to-save-environment-and-money-when-using-github-actions>.

For the linting & formatting action, the concurrency group is set to
contain the PR (if any) and ref otherwise, on the basis that only the
latest push to a PR or branch is interesting.

For the GitHub Pages and Godot Asset Library actions, each run
overwrites the result of any previous run, so the concurrency group is
just the workflow name.

.github/workflows/checks.yaml

@@ -6,6 +6,11 @@ on:
     branches:
       - main
 
+# Cancel any ongoing previous run if a PR is updated
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   pre-commit:
     name: Linting and Formatting

.github/workflows/github-pages.yml

@@ -5,6 +5,11 @@ on:
     types:
       - published
 
+# Cancel any ongoing previous run if the job is re-triggered
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: true
+
 permissions:
   contents: read
   pages: write

.github/workflows/godot-asset-library.yaml

@@ -3,6 +3,11 @@ on:
     types:
       - published
 
+# Cancel any ongoing previous run if the job is re-triggered
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: true
+
 name: Push to Godot Asset Library
 
 jobs: