fix failing kokoro-windows test

[alicejli]

kokoro-windows test started failing June 20th 2023:

[INFO] Running com.google.cloud.tools.appengine.AppEngineDescriptorTest
[Fatal Error] :1:10: DOCTYPE is disallowed when the feature "[http://apache.org/xml/features/disallow-doctype-decl](https://www.google.com/url?q=http://apache.org/xml/features/disallow-doctype-decl&sa=D)" set to true.
[INFO] Tests run: 18, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.346 s - in com.google.cloud.tools.appengine.AppEngineDescriptorTest
[INFO] Running com.google.cloud.tools.appengine.configuration.AppEngineWebXmlProjectStageConfigurationTest
[ERROR] Tests run: 3, Failures: 0, Errors: 3, Skipped: 0, Time elapsed: 0.05 s <<< FAILURE! - in com.google.cloud.tools.appengine.configuration.AppEngineWebXmlProjectStageConfigurationTest
[ERROR] testGetStagingDirectory(com.google.cloud.tools.appengine.configuration.AppEngineWebXmlProjectStageConfigurationTest)  Time elapsed: 0.049 s  <<< ERROR!
java.lang.UnsupportedOperationException: 'posix:permissions' not supported as initial attribute
	at com.google.cloud.tools.appengine.configuration.AppEngineWebXmlProjectStageConfigurationTest.<init>(AppEngineWebXmlProjectStageConfigurationTest.java:30)

[ERROR] testDefaultsToNull(com.google.cloud.tools.appengine.configuration.AppEngineWebXmlProjectStageConfigurationTest)  Time elapsed: 0 s  <<< ERROR!
java.lang.UnsupportedOperationException: 'posix:permissions' not supported as initial attribute
	at com.google.cloud.tools.appengine.configuration.AppEngineWebXmlProjectStageConfigurationTest.<init>(AppEngineWebXmlProjectStageConfigurationTest.java:30)

[ERROR] testGetSourceDirectory(com.google.cloud.tools.appengine.configuration.AppEngineWebXmlProjectStageConfigurationTest)  Time elapsed: 0 s  <<< ERROR!
java.lang.UnsupportedOperationException: 'posix:permissions' not supported as initial attribute
	at com.google.cloud.tools.appengine.configuration.AppEngineWebXmlProjectStageConfigurationTest.<init>(AppEngineWebXmlProjectStageConfigurationTest.java:30)

This is blocking PRs from being merged. I haven't done much digging, but I expect something in the test will need to be updated - I do not think this is related to any recent dependency update as far as I could tell.