ci(kms): build-lux-kms runs-on lux-build (ARC scale-set label)

runs-on [self-hosted,linux,amd64] never matches the lux-build ARC
AutoscalingRunnerSet (its runner label is the scale-set name 'lux-build'),
so Build KMS jobs queued forever with 'assigned job=0'. Same fix already
applied to build-operator.yml.

.github/workflows/build-lux-kms.yml

@@ -17,7 +17,7 @@ permissions:
 
 jobs:
   build:
-    runs-on: [self-hosted, linux, amd64]
+    runs-on: lux-build
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v4