Update RelativeCI agent action (openhab#2063)

The RelativeCI action was using the CLI with an inline API key.

This PR replaces the RelativeCI CLI with the action agent running on workflow_run
event and in the context of the default branch (secrets are not leaked to forks). 

Signed-off-by: Viorel Cojocaru <[email protected]>

.github/workflows/mainui.yml

@@ -74,13 +74,9 @@ jobs:
         with:
           name: report
           path: /home/runner/work/openhab-webui/openhab-webui/bundles/org.openhab.ui/web/report.html
-
-      - name: Upload stats to RelativeCI
-        run: npx relative-ci-agent
-        env:
-          # was: RELATIVE_CI_KEY: ${{ secrets.RELATIVE_CI_KEY }}
-          # but wouldn't work for PRs made from forks...
-          # Bad practice (!), however since the service itself as well as
-          # what can be done with the key (add new bundle stats) is not
-          # critical, not hiding the key is deemed acceptable for now!
-          RELATIVE_CI_KEY: f8e0e450-804c-11eb-b7f5-510e2634a5c9
+
+      # Upload bundle stats to use on relative-ci.yaml workflow
+      - name: Upload webpack stats artifact
+        uses: relative-ci/[email protected]
+        with:
+          webpackStatsFile: ./bundles/org.openhab.ui/web/stats.json

.github/workflows/relative-ci.yml

@@ -0,0 +1,17 @@
+name: RelativeCI
+
+on:
+  workflow_run:
+    workflows: ["Main UI"]
+    types:
+      - completed
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Send bundle stats and build information to RelativeCI
+        uses: relative-ci/[email protected]
+        with:
+          key: ${{ secrets.RELATIVE_CI_KEY }}
+          token: ${{ secrets.GITHUB_TOKEN }}