feat(ci): migrate all workflows to GKE self-hosted runners

Swap all `runs-on: ubuntu-latest` to `runs-on: arc-runner-set` all jobs.
The ARM docker build job in docker.yml uses `${{ matrix.runner }}`
and is left untouched since the GKE runner pool is x86_64 only.

Runners are backed by ARC (Actions Runner Controller) on a GKE cluster
with a spot preemptible node pool that scales based on job demand.

Use the baked Electron dependencies for the desktop E2E job.

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
   # ─────────────────────────────────────────────────────────────────────
   detect:
     name: Detect affected areas
-    runs-on: ubuntu-latest
+    runs-on: arc-runner-set
     timeout-minutes: 10
     outputs:
       python: ${{ steps.classify.outputs.python }}
@@ -181,7 +181,7 @@ jobs:
     name: CI review comment (live)
     needs: [detect, review-labels, lockfile-diff, supply-chain, osv-scanner, uv-lockfile, history-check, contributor-check, e2e-desktop]
     if: always() && github.event_name == 'pull_request' && github.event.pull_request.head.repo.fork != true
-    runs-on: ubuntu-latest
+    runs-on: arc-runner-set
     timeout-minutes: 40
     steps:
       - name: Checkout code
@@ -274,7 +274,7 @@ jobs:
       # we don't require docker to pass rn because it's so slow lol
       # - docker
     if: always()
-    runs-on: ubuntu-latest
+    runs-on: arc-runner-set
     timeout-minutes: 10
     outputs:
       needs-json: ${{ steps.evaluate.outputs.needs-json }}
@@ -316,7 +316,7 @@ jobs:
     name: CI timing report
     needs: [all-checks-pass, docker]
     if: always()
-    runs-on: ubuntu-latest
+    runs-on: arc-runner-set
     timeout-minutes: 10
     steps:
       - name: Checkout code