solver: split platform specific code

Signed-off-by: Sam Alba <samalba@users.noreply.github.com>

solver/socketprovider_windows.go

@@ -0,0 +1,14 @@
+//go:build windows
+// +build windows
+
+package solver
+
+func dialStream(id string) (net.Conn, error) {
+	if !strings.HasPrefix(id, unixPrefix) {
+		return nil, fmt.Errorf("invalid socket forward key %s", id)
+	}
+
+	id = strings.TrimPrefix(id, npipePrefix)
+	dur := time.Second
+	return winio.DialPipe(id, &dur)
+}