Fix Jenkins pipelines on Linux agent

The Linux agent now runs Docker in rootless mode for additional
security. Operations inside containers must be run as root to match the
underprivileged user running Docker.

build/jenkins/pipelines/linux.Jenkinsfile

@@ -1,4 +1,4 @@
-/* Copyright (C) 2024 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -57,6 +57,9 @@ pipeline {
 						customWorkspace "workspace/${JENKINS_COMPILER}-pch"
 						dir 'build/jenkins/dockerfiles'
 						filename "${JENKINS_COMPILER}.Dockerfile"
+						// Prevent Jenkins from running commands with the UID of the host's jenkins user
+						// https://stackoverflow.com/a/42822143
+						args '-u root'
 					}
 				}