wcow: scope security descriptor for the npipe and make it configurable

[profnandaa]

We need to check if this isn't relaxing security too much for default value and implement --group per #4864 (comment) , but I guess for testing experimental feature this is fine for now.

Originally posted by @tonistiigi in #4872 (review)

--

accesschk.exe -d \\.\pipe\\buildkitd

Accesschk v6.15 - Reports effective permissions for securable objects
Copyright (C) 2006-2022 Mark Russinovich
Sysinternals - www.sysinternals.com

\\.\pipe\buildkitd
  RW NT AUTHORITY\Authenticated Users
  RW NT AUTHORITY\SYSTEM
  RW BUILTIN\Administrators

buildkit/cmd/buildkitd/main_windows.go

Lines 23 to 30 in 0cf1004

	pc := &winio.PipeConfig{
	// Allow generic read and generic write access to authenticated users
	// and system users. On Linux, this pipe seems to be given rw access to
	// user, group and others (666).
	// TODO(gabriel-samfira): should we restrict access to this pipe to just
	// authenticated users? Or Administrators group?
	SecurityDescriptor: "D:P(A;;GRGW;;;AU)(A;;GRGW;;;SY)",
	}

[profnandaa]

Cross linking - #4864 (comment)

[profnandaa]

fixed in #4875