[rush] Fix ENOENT in AutoinstallerPluginLoader when destination folder doesn't exist

[Copilot]

PR #5678 switched from FileSystem.copyFile() to readFile() + writeFile() for line-ending normalization, but unlike fs-extra.copySync, writeFile() doesn't create missing parent directories. This causes ENOENT when the per-package subfolder under rush-plugins/ hasn't been created yet.

Changes

• AutoinstallerPluginLoader.ts — add ensureFolderExists: true to both writeFile() calls (manifest and command-line.json):

FileSystem.writeFile(destinationManifestPath, manifestContent, {
  convertLineEndings: NewlineKind.Lf,
  ensureFolderExists: true
});

• Change file — patch bump for @microsoft/rush

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