internal: add Shopify to list of broken auth providers

Shopify uses URLs in the form my-shop-id.myshopify.com as the endpoints
for its OAuth2 dances.

Change-Id: I73d98ca285991b2f73dd9d8d366d3fdbe249e741
Reviewed-on: https://go-review.googlesource.com/42630
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>

internal/token.go

@@ -125,6 +125,7 @@ var brokenAuthHeaderProviders = []string{
 // brokenAuthHeaderDomains lists broken providers that issue dynamic endpoints.
 var brokenAuthHeaderDomains = []string{
 	".force.com",
+	".myshopify.com",
 	".okta.com",
 	".oktapreview.com",
 }