[#60058920] Install Vegeta for Router

[Vegeta](https://github.com/tsenart/vegeta) is an HTTP load testing tool
used by the Router to test for performance regressions. It has been added to
the GOV.UK PPA here:

- https://github.com/alphagov/packager/pull/13
- https://launchpad.net/~gds/+archive/govuk/+sourcepub/3643228/+listing-archive-extra