fix(compiler-cli): normalize mock Windows file paths correctly (#36968)

Since the `MockFileSystemWindows` is case-insensitive, any
drive path that must be added to a normalized path should be lower
case to make the path canonical.

PR Close #36968

packages/compiler-cli/src/ngtsc/file_system/testing/src/mock_file_system_windows.ts

@@ -42,6 +42,6 @@ export class MockFileSystemWindows extends MockFileSystem {
   }
 
   normalize<T extends PathString>(path: T): T {
-    return path.replace(/^[\/\\]/i, 'C:/').replace(/\\/g, '/') as T;
+    return path.replace(/^[\/\\]/i, 'c:/').replace(/\\/g, '/') as T;
   }
 }