Replace ${{vars.RUNS_ON}} with ubuntu-latest in workflow files (#1176)

Author: Copilot

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ defaults:
 jobs:
   check-formatting-dockerfile:
     name: Check Formatting (Dockerfile)
-    runs-on: ${{vars.RUNS_ON}}
+    runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:
       - name: Checkout
@@ -15,7 +15,7 @@ jobs:
         uses: hadolint/hadolint-action@v3.3.0
   check-formatting-markdown:
     name: Check Formatting (Markdown)
-    runs-on: ${{vars.RUNS_ON}}
+    runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:
       - name: Checkout
@@ -26,7 +26,7 @@ jobs:
           globs: '**/*.md'
   build-and-test:
     name: Build & Test
-    runs-on: ${{vars.RUNS_ON}}
+    runs-on: ubuntu-latest
     timeout-minutes: 30
     services:
       registry:

.github/workflows/pull-request.yml

@@ -13,7 +13,7 @@ jobs:
     uses: ./.github/workflows/ci.yml
   label:
     name: Label
-    runs-on: ${{vars.RUNS_ON}}
+    runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:
       - name: Apply labels

.github/workflows/release.yml

@@ -13,7 +13,7 @@ jobs:
     uses: ./.github/workflows/ci.yml
   cd:
     name: CD
-    runs-on: ${{vars.RUNS_ON}}
+    runs-on: ubuntu-latest
     timeout-minutes: 30
     if: (!contains(github.event.head_commit.message, '[skip release]'))
     permissions: