os: support deleting read-only files in RemoveAll on older Windows versions
The Windows implementation of RemoveAll supports deleting read-only
files only on file systems that supports POSIX semantics and on
newer Windows versions (Windows 10 RS5 and latter).
For all the other cases, the read-only bit was not clearer before
deleting read-only files, so they fail to delete.
Note that this case was supported prior to CL 75922, which landed on
Go 1.25.
Fixes #75922
Change-Id: Id6e6477f42e1952d08318ca3e4ab7c1648969f66
Reviewed-on: https://go-review.googlesource.com/c/go/+/713480
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>