Investigate potentially failing `test_should_set_chmod()` test

[swissspidy]

This is a follow-up to #57774, which added a suite of new tests in 6.5. Since that milestone is soon closing, I am opening this to further investigate the \Tests_Filesystem_WpFilesystemDirect_Mkdir::test_should_set_chmod() test, which seems to fail on a certain host. See ​https://make.wordpress.org/hosting/test-results/r57860/wpmudevbot-r57860/

This could either be an issue with the host, core, or simply the test itself.

This test is also the last one with an is_windows() check. Would be good if we could get rid of this env-specific check and maybe rely on comparing with FS_CHMOD_DIR instead.