Resolve "Only run `nilaway` CI job when there's actual `.go` changes"

This MR introduces a new .rules:go-changes definition:

.rules:go-changes:
  rules:
    - changes:
        - 'go.mod'
        - 'go.sum'
        - '**/*.go'

that is used by the .test-job definition (so for all jobs in the test stage) as well as lint:nilaway.

Closes: Only run `nilaway` CI job when there's actual `... (#778 - closed)