chore: fix GHA concurrency check to not apply to main branch (#771)

.github/workflows/ci.yaml

diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 3179533..d2ca737 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -12,9 +12,10 @@
   workflow_dispatch:
 
 concurrency:
-  # Cancel previous actions from the same PR: https://stackoverflow.com/a/72408109
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
-  cancel-in-progress: true
+  # Cancel previous actions from the same PR or branch except 'main' branch.
+  # See https://docs.github.com/en/actions/using-jobs/using-concurrency and https://docs.github.com/en/actions/learn-github-actions/contexts for more info.
+  group: concurrency-group::${{ github.workflow }}::${{ github.event.pull_request.number > 0 && format('pr-{0}', github.event.pull_request.number) || github.ref_name }}${{ github.ref_name == 'main' && format('::{0}', github.run_id) || ''}}
+  cancel-in-progress: ${{ github.ref_name != 'main' }}
 
 jobs:
   # matrix-prep-* steps dynamically generate a bit of JSON depending on whether our action has