fix(test): add pnpm-workspace.yaml to multiple-scripts-error-exit fixture

The errorHandler test runs pnpm from this fixture directory. Without its
own workspace manifest, pnpm walked up and discovered the monorepo root,
triggering verifyDepsBeforeRun which modified unrelated files.

__fixtures__/multiple-scripts-error-exit/pnpm-workspace.yaml

@@ -0,0 +1 @@
+{}