Flaky spec/lib/gitlab/secret_detection/scan_spec.rb (gitlab-secret_detection gem)

From https://gitlab.com/gitlab-org/gitlab/-/jobs/6035535488:

Failures:
  1) Gitlab::SecretDetection::Scan when matching patterns when configured with time out whole secret detection scan operation times out
     Failure/Error:
       found_secrets = Parallel.flat_map(
         grouped_blobs,
         in_processes: MAX_PROCS_PER_REQUEST,
         isolation: true # do not reuse sub-processes
       ) do |grouped_blob|
         grouped_blob.flat_map do |blob|
           Timeout.timeout(blob_timeout) do
             find_secrets(blob)
           end
         rescue Timeout::Error => e
     ArgumentError:
       bad signal type NilClass
     # ./vendor/ruby/3.2.0/gems/parallel-1.24.0/lib/parallel.rb:206:in `trap'
     # ./vendor/ruby/3.2.0/gems/parallel-1.24.0/lib/parallel.rb:206:in `restore_interrupt'
     # ./vendor/ruby/3.2.0/gems/parallel-1.24.0/lib/parallel.rb:178:in `ensure in kill_on_ctrl_c'
     # ./vendor/ruby/3.2.0/gems/parallel-1.24.0/lib/parallel.rb:178:in `kill_on_ctrl_c'
     # ./vendor/ruby/3.2.0/gems/parallel-1.24.0/lib/parallel.rb:502:in `work_in_processes'
     # ./vendor/ruby/3.2.0/gems/parallel-1.24.0/lib/parallel.rb:291:in `map'
     # ./vendor/ruby/3.2.0/gems/parallel-1.24.0/lib/parallel.rb:304:in `flat_map'
     # ./lib/gitlab/secret_detection/scan.rb:178:in `run_scan_within_subprocess'
     # ./lib/gitlab/secret_detection/scan.rb:89:in `block in secrets_scan'
     # ./lib/gitlab/secret_detection/scan.rb:83:in `secrets_scan'
     # ./spec/lib/gitlab/secret_detection/scan_spec.rb:257:in `block (4 levels) in <top (required)>'
Finished in 50.45 seconds (files took 0.28728 seconds to load)
13 examples, 1 failure
Failed examples:
rspec ./spec/lib/gitlab/secret_detection/scan_spec.rb:252 # Gitlab::SecretDetection::Scan when matching patterns when configured with time out whole secret detection scan operation times out

/cc: @vbhat161