Fix PR build concurrency and stop auto assigning reviewers for draft PRs (#5519)

* build: Remove concurrency

It's called by checks anyway.

* Only assign reviewers for PRs that are ready for reviews

.github/workflows/build.yml

@@ -3,10 +3,6 @@ name: Build job
 on:
   workflow_call:
 
-concurrency:
-  group: pr-builds-${{ github.event.number }}
-  cancel-in-progress: true
-
 env:
   POWERSHELL_TELEMETRY_OPTOUT: 1
   DOTNET_CLI_TELEMETRY_OPTOUT: 1