Use admin_importmaps branch of Alchemy

We need this PR

AlchemyCMS/alchemy_cms#2884

for now to be able to use the `Alchemy.importmap`
for the `alchemy-product-select`.

Gemfile

@@ -11,7 +11,7 @@ end
 
 alchemy_version = ENV.fetch("ALCHEMY_VERSION", "main")
 if alchemy_version == "main"
-  gem "alchemy_cms", github: "AlchemyCMS/alchemy_cms", branch: "main"
+  gem "alchemy_cms", github: "AlchemyCMS/alchemy_cms", branch: "admin_importmaps"
 else
   gem "alchemy_cms", "~> #{alchemy_version}"
 end