ci(regression): add concurrency group to cancel superseded runs (heygen-com#425)

Matches the pattern already in place on ci.yml, docs.yml,
windows-render.yml, and catalog-previews.yml. The regression workflow
was the only one without it.

Without this, rapid pushes to a PR leave prior regression runs still
executing their full matrix (~10 parallel shards across styles-a..g,
fast, render-compat, hdr) even though they'll be thrown away. On a busy
day this alone can eat a double-digit share of the GitHub hosted runner
pool and stretch queues for every open PR.

Author: jrusso1020

.github/workflows/regression.yml

@@ -6,6 +6,10 @@ on:
     branches:
       - main
 
+concurrency:
+  group: regression-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   changes:
     name: Detect changes