Trust the self-signed certificate for gem url

Chef Infra (archive)
1

Hi there, how can I trust my self-signed certificate for gem url used in chef_gem resource?
I have the certificate in the /etc/chef/trusted_certs folder on chef-client, but it doesn't work.

test.rb

chef_gem 'mixlib-install' do
  compile_time false
  action :install
  compile_time true
  source "https://<domainname>/artifactory/api/gems/gems"
end

# ls -lh /etc/chef/trusted_certs
<domainname>.crt

# chef-apply test.rb -w -c /etc/chef/client.rb
Recipe: (chef-apply cookbook)::(chef-apply recipe)
  * chef_gem[mixlib-install] action installERROR:  SSL verification error at depth 2: self signed certificate in certificate chain (19)
ERROR:  Root certificate is not trusted (/CN=MetLife Root Certificate Authority)
ERROR:  SSL verification error at depth 2: self signed certificate in certificate chain (19)
ERROR:  Root certificate is not trusted (/CN=MetLife Root Certificate Authority)
ERROR:  SSL verification error at depth 2: self signed certificate in certificate chain (19)
ERROR:  Root certificate is not trusted (/CN=MetLife Root Certificate Authority)
ERROR:  SSL verification error at depth 1: self signed certificate in certificate chain (19)
ERROR:  Root certificate is not trusted (/CN=PDC-internet-decrypt)
ERROR:  SSL verification error at depth 1: self signed certificate in certificate chain (19)
ERROR:  Root certificate is not trusted (/CN=PDC-internet-decrypt)
ERROR:  SSL verification error at depth 1: self signed certificate in certificate chain (19)
ERROR:  Root certificate is not trusted (/CN=PDC-internet-decrypt)
[2020-01-09T14:11:24+09:00] WARN: failed to find gem mixlib-install (>= 0) from [https://<domainname>/artifactory/api/gems/gems
, https://www.rubygems.org]