change yaml style to follow GHA style

.github/workflows/build_test.yaml

@@ -1,6 +1,7 @@
 ---
 name: Validate rosdistro
-"on":
+# this is fine since gha runs with yaml 1.2
+on: # yamllint disable-line rule:truthy
  push:
  branches: ['master']
  pull_request:

.github/workflows/labeler.yaml

@@ -1,6 +1,7 @@
 ---
 name: "Pull Request Labeler"
-"on":
+# this is fine since gha runs with yaml 1.2
+on: # yamllint disable-line rule:truthy
 - pull_request_target
 
 permissions:

.github/workflows/stale_labeler.yaml

@@ -1,7 +1,8 @@
 ---
 name: 'Label inactive PRs as stale'
 
-"on":
+# this is fine since gha runs with yaml 1.2
+on: # yamllint disable-line rule:truthy
  schedule:
  - cron: '0 11 * * *' # runs at 11am UTC every day => early morning in US