Fix MIGRATE_PIPELINE_ROOT path inside container

- Set MIGRATE_PIPELINE_ROOT=/workspace in container script heredoc
- Host path /home/torsten/sandbox/qt6-migration-pipeline maps to /workspace in container
- Fixes jenkins_builder_ubuntu26.sh not found error

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/new_docker_build.sh

@@ -1309,6 +1309,7 @@ INTEGRATION_TEST="${INTEGRATION_TEST:-OFF}"
 APT_PACKAGES_STRING="${APT_PACKAGES_STRING:-}"
 # Jenkins builder and packaging variables
 RUN_JENKINS_BUILDER="${RUN_JENKINS_BUILDER:-OFF}"
+MIGRATE_PIPELINE_ROOT="/workspace"
 PACKAGE_OUTPUT_DIR="${PACKAGE_OUTPUT_DIR:-/workspace/packages-out}"
 LTS_VERSION="${LTS_VERSION:-1.0}"
 PROJECT="${PROJECT:-guez}"
@@ -1339,6 +1340,14 @@ log_info() {
   printf 'INFO: %s\n' "$*"
 }
 
+log_error() {
+  printf 'ERROR: %s\n' "$*" >&2
+}
+
+log_warn() {
+  printf 'WARNING: %s\n' "$*" >&2
+}
+
 install_qt6_packages() {
   log_info "Installing Qt6 packages via aqtinstall"