Use new syntax to set environment variable

Use new syntax to set environment variable value.
See https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#setting-an-environment-variable.

Author: martincostello

.github/workflows/build.yml

@@ -48,7 +48,7 @@ jobs:
         $QuarterHours = [Math]::Floor($Now.Minute / 15.0)
         $Revision = $Hours + $QuarterHours + 1
         $BuildId = $Now.ToString("yyyyMMdd") + "." + $Revision
-        Write-Host "::set-env name=_AspNetContribBuildNumber::${BuildId}"
+        Write-Host "_AspNetContribBuildNumber=${BuildId}" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
 
     - name: Build, Test and Package
       if: ${{ runner.os == 'Windows' }}