When executing a process with the executeProcess Operator the default working directory is the home directory of the user that started Nifi. In the code there is a Working directory Property that seems to work if one adds it. The documentation does not mention that it reads that argument and it is not shown in the Properties of the Operator.
It should either be added in the documentation of the operator or directly in the Properties with home directory as the default value.
See Line 100 to 110 in https://github.com/apache/nifi/blob/6279fd41844205a14cf1f2fb225ead8881b197eb/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/ExecuteProcess.java