fix(plugins, utils): Plugin admin folder loading with backslash on Windows (#12418)

* fix: Plugin admin folder loading with backslash on Windows

* fix: Plugin admin folder loading with backslash on Windows - Add changeset

---------

Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

.changeset/thin-buckets-behave.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/utils": patch
+---
+
+fix(plugins): plugin admin folder loading with backslash on Windows

packages/core/utils/src/common/get-resolved-plugins.ts

@@ -97,7 +97,7 @@ async function resolvePlugin(
   const adminConfig = hasAdmin
     ? {
         type: isAdminLocal ? ("local" as const) : ("package" as const),
-        resolve: path.join(
+        resolve: path.posix.join(
           isAdminLocal ? pluginStaticOptions.srcDir : name,
           "admin"
         ),