CI: add concurrency group to E2E workflow to cancel redundant runs

When spr force-pushes all stack branches at once, each branch triggers
a new E2E run while the previous one is still queued. The concurrency
group (keyed on branch ref) cancels the stale run, cutting the number
of concurrent E2E jobs roughly in half.

commit-id:1a8eb714

Author: jucor

.github/workflows/cypress-tests.yml

@@ -9,6 +9,10 @@ on:
       - stable
       - 'jc/**'
 
+concurrency:
+  group: e2e-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   cypress-run:
     runs-on: ubuntu-latest

.spr.yml

@@ -0,0 +1,14 @@
+githubRepoOwner: compdemocracy
+githubRepoName: polis
+githubHost: github.com
+githubRemote: origin
+githubBranch: edge
+requireChecks: true
+requireApproval: true
+defaultReviewers: []
+mergeMethod: squash
+mergeQueue: false
+prTemplateType: stack
+forceFetchTags: false
+showPrTitlesInStack: false
+branchPushIndividually: false