github: cancel in-progress builds on stale PRs

When a pull request is pushed multiple times in sequence (e.g. to fix small
errors that are noticed post-submission), a backlog of builds ends up needing
to be cleared out before your new update can be built. If you push N times, N
builds are queued and need to clear out first.

This can cause higher latency for *every* PR since the pool of public runners
is shared, and in general seems uneconomical and inefficient since 99% of the
time you want to build the new version ASAP.

Luckily GHA has a universal solution to this: use the `concurrency` directive
to group all builds for a PR under a name, and when a new build appears in that
group, cancel all builds in the group that are in-progress.

Taken from this useful blog post: "Simple trick to save environment and money
when using GitHub Actions"

https://turso.tech/blog/simple-trick-to-save-environment-and-money-when-using-github-actions

Signed-off-by: Austin Seipp <[email protected]>

.github/workflows/build-nix.yml

@@ -6,6 +6,10 @@ on:
       - main
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 permissions: read-all
 
 jobs:

.github/workflows/build.yml

@@ -4,6 +4,10 @@ on:
   push:
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 permissions: read-all
 
 env:

.github/workflows/codespell.yml

@@ -6,6 +6,10 @@ on:
       - main
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 permissions: read-all
 
 jobs:

.github/workflows/dependabot.yml

@@ -3,6 +3,10 @@ name: Enable auto-merge for Dependabot PRs
 on:
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 permissions: read-all
 
 jobs: