Run brakeman checks in local build

This is already running as part of the CI build [1] and I've been caught
out before when the local build passed, but the CI build failed which is
sub-optimal. I copied the actual command from the one that runs in the
CI build [2].

Adding this to the `lint` rake task means that it will run when you run
the default rake task locally, but it won't run twice in CI, because the
latter doesn't run the default rake task.

[1]: https://github.com/alphagov/signon/blob/acae02f346e3880368f47622fcad2aa80558eb42/.github/workflows/ci.yml#L14-L16
[2]: https://github.com/alphagov/govuk-infrastructure/blob/6b6fe37cfd6af8ef57eff983e4356e96f7b3fd14/.github/workflows/brakeman.yml#L20C14-L20C71

lib/tasks/lint.rake

@@ -1,5 +1,6 @@
 desc "Run all linters"
 task lint: :environment do
   sh "yarn run lint"
+  sh "bundle exec brakeman . --except CheckRenderInline --quiet"
   sh "bundle exec rubocop"
 end