Skip signed plugin build on pull requests

A pull request from a fork is not able to build the signed plugin due to
the secret API KEY, so this step should be skipped. In other instances,
like releases and daily builds, the signed plugin should be created.

.github/workflows/ci.yml

@@ -74,6 +74,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Build frontend signed
+        if: (github.event_name != 'pull_request') || (github.event.pull_request.head.repo.full_name == github.repository)
         id: build-signed
         uses: ./.github/build/
         env:
@@ -83,6 +84,7 @@ jobs:
           signed: true
 
   validate-plugin:
+    if: (github.event_name != 'pull_request') || (github.event.pull_request.head.repo.full_name == github.repository)
     runs-on: ubuntu-latest
     needs: build
     steps: