fix: update runner label from local-ubuntu-latest to runners-privilegedescalation

ARC runner scale set was recreated on 2026-03-19 with label
`runners-privilegedescalation` but all shared workflows still referenced
`local-ubuntu-latest`. This label mismatch caused startup_failure on
every Release workflow and queued CI jobs with no runner to pick them up.

Updates all 4 workflows and the actionlint config to match the current
ARC runner scale set label.

Closes #27

.github/actionlint.yaml

@@ -1,3 +1,3 @@
 self-hosted-runner:
   labels:
-    - local-ubuntu-latest
+    - runners-privilegedescalation

.github/workflows/ci-health-check.yaml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   health-check:
-    runs-on: local-ubuntu-latest
+    runs-on: runners-privilegedescalation
     steps:
       - name: Checkout repository
         uses: actions/checkout@v6

.github/workflows/plugin-ci.yaml

@@ -11,7 +11,7 @@ on:
 
 jobs:
   ci:
-    runs-on: local-ubuntu-latest
+    runs-on: runners-privilegedescalation
     timeout-minutes: 10
 
     steps:

.github/workflows/plugin-release.yaml

@@ -34,7 +34,7 @@ jobs:
 
   release:
     needs: ci
-    runs-on: local-ubuntu-latest
+    runs-on: runners-privilegedescalation
     timeout-minutes: 10
 
     steps:

.github/workflows/pr-validation.yaml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   validate:
-    runs-on: local-ubuntu-latest
+    runs-on: runners-privilegedescalation
     timeout-minutes: 5
 
     steps: