[Github] Explicitly specify runner groups

.github/workflows/premerge.yaml

@@ -28,7 +28,23 @@ jobs:
     if: >-
         github.repository_owner == 'llvm' &&
         (github.event_name != 'pull_request' || github.event.action != 'closed')
-    runs-on: llvm-premerge-linux-runners
+    # We explicitly distribute jobs between the us-west and us-central clusters
+    # because the racey assignment that Github ARC should be doing automatically
+    # is not currently working.
+    # TODO(boomanaiden154): Remove this once we have finished investigating and
+    # have fixed the Github ARC scheduling.
+    runs-on:
+      group: ${{ (
+                    endsWith(github.run_id, '0') ||
+                    endsWith(github.run_id, '2') ||
+                    endsWith(github.run_id, '4') ||
+                    endsWith(github.run_id, '6') ||
+                    endsWith(github.run_id, '8')
+                 ) && 
+                 'llvm-premerge-cluster-us-west' || 
+                 'llvm-premerge-cluster-us-central'
+              }}
+      labels: llvm-premerge-linux-runners
     steps:
       - name: Checkout LLVM
         uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
@@ -88,7 +104,23 @@ jobs:
     if: >-
         github.repository_owner == 'llvm' &&
         (github.event_name != 'pull_request' || github.event.action != 'closed')
-    runs-on: llvm-premerge-windows-2022-runners
+    # We explicitly distribute jobs between the us-west and us-central clusters
+    # because the racey assignment that Github ARC should be doing automatically
+    # is not currently working.
+    # TODO(boomanaiden154): Remove this once we have finished investigating and
+    # have fixed the Github ARC scheduling.
+    runs-on:
+      group: ${{ (
+                    endsWith(github.run_id, '0') ||
+                    endsWith(github.run_id, '2') ||
+                    endsWith(github.run_id, '4') ||
+                    endsWith(github.run_id, '6') ||
+                    endsWith(github.run_id, '8')
+                 ) && 
+                 'llvm-premerge-cluster-us-west' || 
+                 'llvm-premerge-cluster-us-central'
+              }}
+      labels: llvm-premerge-windows-2022-runners
     defaults:
       run:
         shell: bash