solver: split platform specific code

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

solver/socketprovider_unix.go

@@ -0,0 +1,20 @@
+//go:build !windows
+// +build !windows
+
+package solver
+
+import (
+	"fmt"
+	"net"
+	"strings"
+	"time"
+)
+
+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, unixPrefix)
+	return net.DialTimeout("unix", id, time.Second)
+}