Fix build id variable name when selecting CI container (PowerShell#17279)

Author: TravisEz13

.vsts-ci/linux.yml

@@ -116,11 +116,13 @@ stages:
       clean: true
 
     - pwsh: |
+        $buildId = $env:BUILD_BUILDID
+        Write-Verbose "Fall back seed: $buildId" -Verbose
         # For PRs set the seed to the PR number so that the image is always the same
         $seed = $env:SYSTEM_PULLREQUEST_PULLREQUESTID
         if(!$seed) {
           # for non-PRs use the integer identifier of the build as the seed.
-          $seed = $env:SYSTEM_BUILD_BUILDID
+          $seed = $buildId
         }
         Write-Verbose "Seed: $seed" -Verbose