fix TestLockedByOthers on Windows

[djdv]

This test was failing on Windows since both the value's being checked are not the same ones used on this platform.
Specifically, this causes

errors.As(err, new(fslock.LockedError))

to always be false (on Windows).

The Is check I have in this patch is valid, but currently useless since we'll never receive that concrete value from the lock library.
The string check should catch this error regardless.

I'm going to submit a patch to the lock library and see what they think about wrapping the error value.
We can wait on that and maybe include a dependency update with this PR if it goes through.

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[djdv]

Suggested change

	return errors.Is(err, windows.ERROR_SHARING_VIOLATION) || strings.Contains(err.Error(), "being used by another process")
	return errors.Is(err, windows.ERROR_SHARING_VIOLATION) || strings.Contains(err.Error(), windows.ERROR_SHARING_VIOLATION.Error())

[djdv]

^this pattern should probably be repeated for the other platforms as well.
That is, in the _posix.go file, don't use the equality operator to check syscall error values, instead use errors.Is with them; and call their Error method too.
If the Unix error value on the lefthand of the OR isn't already matched up with the one we expect to receive from lock.Lock we can update it.

As is, I don't think the lefthand condition could ever be true, since Lock (currently) returns a string error, never a syscall.Errno.