[TypeSpec Authoring][Security] Remove all npm install -g usage in pipelines

Parent epic: #15730

Remove all `npm install -g` usage from pipelines. Pending on engsys best practice.

Why:
- Global installs cannot lock package.

Scope:
- azure-typespec-author benchmark pipeline

Start by locating the azure-typespec-author benchmark pipeline and review the pending engsys best practice. Remove its npm install -g usage so the pipeline installs lockable dependencies, then verify that no global npm install remains in the scoped pipeline.

Written by the indexing model from the issue text.