fix: use client preferred URL for the default DERP (#18911)

The agentsdk currently does a remap of the DERP map to change the
EmbeddedRelay node's URL to match the agent's access URL.

This PR makes changes to the `workspacesdk` (used by clients like the
CLI) and `vpn` (used by Coder Desktop) to match this behavior.

This enables us the ability to try Coder clients in dogfood over a VPN
without changing the global access URL.

codersdk/workspacesdk/workspacesdk_test.go

@@ -19,7 +19,6 @@ import (
 
 	"github.com/coder/coder/v2/coderd/httpapi"
 	"github.com/coder/coder/v2/codersdk"
-	"github.com/coder/coder/v2/codersdk/agentsdk"
 	"github.com/coder/coder/v2/codersdk/workspacesdk"
 	"github.com/coder/coder/v2/tailnet"
 	"github.com/coder/coder/v2/testutil"
@@ -43,7 +42,7 @@ func TestWorkspaceRewriteDERPMap(t *testing.T) {
 	}
 	parsed, err := url.Parse("https://coconuts.org:44558")
 	require.NoError(t, err)
-	client := agentsdk.New(parsed)
+	client := workspacesdk.New(codersdk.New(parsed))
 	client.RewriteDERPMap(dm)
 	region := dm.Regions[1]
 	require.True(t, region.EmbeddedRelay)