Fix(lib/fs/tests): Avoid permission denials when cleaning up TempDirs in set_get_permissions_nofollows*

[PaulDance]

At least under Windows 7, the set_get_permissions_nofollows and set_get_permissions_nofollows_symlink FS tests currently fail on:

---- fs::tests::set_get_permissions_nofollows stdout ----
thread 'fs::tests::set_get_permissions_nofollows' (2308) panicked at library/std/src/test_helpers.rs:53:20:
called `Result::unwrap()` on an `Err` value: Os { code: 5, kind: PermissionDenied, message: "Access is denied." }
---- fs::tests::set_get_permissions_nofollows stdout end ----
---- fs::tests::set_get_permissions_nofollows_symlink stdout ----
thread 'fs::tests::set_get_permissions_nofollows_symlink' (1108) panicked at library/std/src/test_helpers.rs:53:20:
called `Result::unwrap()` on an `Err` value: Os { code: 5, kind: PermissionDenied, message: "Access is denied." }
---- fs::tests::set_get_permissions_nofollows_symlink stdout end ----

The panic clearly occurs in TempDir::drop that calls fs::remove_dir_all. This is consistent with the fact that FILE_ATTRIBUTE_READONLY is set on the file:

Applications can read the file, but cannot write to it or delete it.

from the attribute's documentation.

This therefore fixes these tests by resetting the attribute before letting the drop guard run.

cc #141607 @roblabla

@rustbot label T-libs A-io O-windows

[rustbot]

r? @clarfonthey

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