github: actions: Use on-PR labeler

Now that the standard GitHub action supports forks, switch to the on-PR
action instead of running a cronjob.

Reference:

https://github.com/actions/labeler/issues/12#issuecomment-670967607

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

.github/workflows/labeler.yml

@@ -0,0 +1,11 @@
+name: 'Pull Request Labeler'
+on:
+  - pull_request_target
+
+jobs:
+  triage:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/labeler@v2.1.1
+        with:
+          repo-token: '${{ secrets.GITHUB_TOKEN }}'

.github/workflows/periodic_labeler.yml

@@ -1,13 +0,0 @@
-name: Pull request labeler
-on:
-  schedule:
-    - cron: '*/5 * * * *'
-jobs:
-  labeler:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: paulfantom/periodic-labeler@master
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          GITHUB_REPOSITORY: ${{ github.repository }}
-          LABEL_MAPPINGS_FILE: .github/labeler.yml