Merge #7219: ci: run check-skip on blacksmith with GitHub-hosted fallback

d02243c ci: run check-skip on blacksmith with GitHub-hosted fallback (PastaClaw)

Pull request description:

  ## Issue

  The `check-skip` job runs on `ubuntu-latest` (GitHub-hosted only). When all GitHub runners are busy, this lightweight job stalls — blocking the entire CI pipeline since every other job depends on it.

  ## Fix

  Use `vars.RUNNER_AMD64` with a fallback to `ubuntu-24.04`:

  ```yaml
  runs-on: ${{ vars.RUNNER_AMD64 || 'ubuntu-24.04' }}
  ```

  - **dashpay org:** `RUNNER_AMD64` is set to a Blacksmith runner label → uses Blacksmith
  - **External forks:** Variable not set → falls back to GitHub-hosted `ubuntu-24.04`

  This is the same pattern already used for build jobs in the repo.

ACKs for top commit:
  UdjinM6:
    utACK d02243c

Tree-SHA512: 63b71fb39d8308937486d3ef1724c86d75267fb9ce1ada618a661732bf78e80f8c32fdfffb6f11ef0d9173f776db815904298745492219388667d58e2935cf0b

Author: PastaPastaPasta

.github/workflows/build.yml

@@ -20,7 +20,7 @@ concurrency:
 jobs:
   check-skip:
     name: Check skip conditions
-    runs-on: ubuntu-latest
+    runs-on: ${{ vars.RUNNER_AMD64 || 'ubuntu-24.04' }}
     outputs:
       skip: ${{ steps.skip-check.outputs.skip }}
     steps: