Update build.yml

https://dev.to/petrsvihlik/using-environment-protection-rules-to-secure-secrets-when-building-external-forks-with-pullrequesttarget-hci

.github/workflows/build.yml

@@ -3,9 +3,20 @@ name: Build-CI
 on: [pull_request, push]
 
 jobs:
+
+  approve: # First step
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Approve
+      run: echo For security reasons, all pull requests to this repository need to be approved first before running any automated CI.
+
   build:
     runs-on: ubuntu-latest
 
+    needs: [approve] # Require the first step to finish
+    environment:
+      name: IO
     steps:
     - uses: actions/checkout@v2