[BUG]: update benchmark workflow for changed files to not run for the entire namespace

[kgryte]

Currently, when you update a namespace file (e.g., @stdlib/ndarray/lib/index.js), benchmarks are run for the entirety of the namespace. This is often not what is wanted. Rather, the benchmarks should be scoped to the namespace package itself. With the growth of certain namespaces, any time we update a namespace file, we end up clogging our workflow queue and are now experiencing timeouts.

[kgryte]

We should probably also make a similar update for tests.