fix: Work around psalm taint false-positive by not using var_export

var_export is listed as a taint sink because it may output stuff
 depending on the parameters. It was not the case here, but we can
 simply json_encode the result by passing it as context to the logger
 method rather than using var_export.

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

lib/private/TaskProcessing/Manager.php

@@ -999,7 +999,7 @@ class Manager implements IManager {
 				$task->setEndedAt(time());
 				$error = 'The task was processed successfully but the provider\'s output doesn\'t pass validation against the task type\'s outputShape spec and/or the provider\'s own optionalOutputShape spec';
 				$task->setErrorMessage($error);
-				$this->logger->error($error . ' Output was: ' . var_export($result, true), ['exception' => $e]);
+				$this->logger->error($error, ['exception' => $e, 'output' => $result]);
 			} catch (NotPermittedException $e) {
 				$task->setProgress(1);
 				$task->setStatus(Task::STATUS_FAILED);