Fix CI (#560)

After #511, CI is failing when merging a PR.
See: https://github.com/quarkiverse/quarkus-openapi-generator/actions/runs/6811912710

.github/workflows/build.yml

@@ -49,10 +49,12 @@ jobs:
         run: mvn '-Dorg.slf4j.simpleLogger.log.org.openapitools=off' -B formatter:validate impsort:check verify --file pom.xml
 
       - name: Store PR id
+        if: github.event_name == 'pull_request'
         run: |
           echo ${{ github.event.number }} > ./docs/target/generated-docs/pr-id.txt
 
       - name: Publishing directory for PR preview
+        if: github.event_name == 'pull_request'
         uses: actions/upload-artifact@v3
         with:
           name: site
@@ -86,12 +88,14 @@ jobs:
         run: mvn -Presteasy-reactive '-Dorg.slf4j.simpleLogger.log.org.openapitools=off' -B formatter:validate impsort:check verify --file pom.xml
 
       - name: Store PR id
+        if: github.event_name == 'pull_request'
         run: |
           echo ${{ github.event.number }} > ./docs/target/generated-docs/pr-id.txt
 
       - name: Publishing directory for PR preview
+        if: github.event_name == 'pull_request'
         uses: actions/upload-artifact@v3
         with:
           name: site
           path: ./docs/target/generated-docs
-          retention-days: 3
+          retention-days: 3