Add build scan tag for all pull request builds (#47889)

gradle/build-scan.gradle

@@ -1,4 +1,3 @@
-import nebula.plugin.info.scm.ScmInfoExtension
 import org.elasticsearch.gradle.OS
 
 buildScan {
@@ -37,6 +36,7 @@ buildScan {
             value 'Git Branch', System.getenv('ghprbTargetBranch')
             tag System.getenv('ghprbTargetBranch')
             tag "pr/${System.getenv('ghprbPullId')}"
+            tag 'pull-request'
             link 'Source', "https://github.com/elastic/elasticsearch/tree/${System.getenv('ghprbActualCommit')}"
             link 'Pull Request', System.getenv('ghprbPullLink')
         } else {