add optional artifact check via external_artifact_check in jenkins_vars.yml

[aptalca]

A few images use a custom external trigger to check for upstream artifacts before triggering a new build. However, once switched over to custom, the whole external trigger is manually managed and any update to the template requires manual updates to the custom ones.

This PR adds the external artifact check into the template, activated a new block var in jenkins-vars.yml named external_artifact_check.

This var can be set to any multiline bash commands (written in the jinja/ansible template style) and it will be injected into the external trigger.
The only requirement is that the var sets artifacts_found to either true or false depending on whatever check needs to be performed.

A sample var for Emby is as follows:

external_artifact_check: |
  assets=$(curl -u "${{ '{{' }} secrets.CR_USER {{ '}}' }}:${{ '{{' }} secrets.CR_PAT {{ '}}' }}" -sX GET "https://api.github.com/repos/MediaBrowser/Emby.Releases/releases/tags/${EXT_RELEASE}" | jq -r '.assets[].browser_download_url')
  if grep -q "emby-server-deb_${EXT_RELEASE}_arm64.deb$" <<< "${assets}" && grep -q "emby-server-deb_${EXT_RELEASE}_amd64.deb$" <<< "${assets}"; then
    artifacts_found="true"
  else
    artifacts_found="false"
  fi

This block gets entered into the external trigger as shown here:
https://github.com/linuxserver/docker-emby/blob/bfa7d0bf3c127146ec5e7209cb6eadd5079b9952/.github/workflows/external_trigger.yml#L80-L85
As you can see, the github workflow variables such as ${{ secrets.CR_USER }} need to be entered into jenkins-vars.yml as ${{ '{{' }} secrets.CR_USER {{ '}}' }} because they go through jinja/ansible templating.

Here's the output of the test trigger: https://github.com/linuxserver/docker-emby/actions/runs/10725065218 (it was manually adjusted to force triggering even on the same version)
Here's the output of the test trigger with artifact check failed intentionally: https://github.com/linuxserver/docker-emby/actions/runs/10725987059

[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/jenkins-builder/1c444d00-pkg-1c444d00-dev-603c50c3583dab079313c667111347b6fc1e86e2-pr-283/index.html
https://ci-tests.linuxserver.io/lspipepr/jenkins-builder/1c444d00-pkg-1c444d00-dev-603c50c3583dab079313c667111347b6fc1e86e2-pr-283/shellcheck-result.xml

Tag	Passed
amd64-1c444d00-pkg-1c444d00-dev-603c50c3583dab079313c667111347b6fc1e86e2-pr-283	✅
arm64v8-1c444d00-pkg-1c444d00-dev-603c50c3583dab079313c667111347b6fc1e86e2-pr-283	✅

[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/jenkins-builder/1c444d00-pkg-1c444d00-dev-9b1ac5e7372a1f53a45a58fddad2178344add5d1-pr-283/index.html
https://ci-tests.linuxserver.io/lspipepr/jenkins-builder/1c444d00-pkg-1c444d00-dev-9b1ac5e7372a1f53a45a58fddad2178344add5d1-pr-283/shellcheck-result.xml

Tag	Passed
amd64-1c444d00-pkg-1c444d00-dev-9b1ac5e7372a1f53a45a58fddad2178344add5d1-pr-283	✅
arm64v8-1c444d00-pkg-1c444d00-dev-9b1ac5e7372a1f53a45a58fddad2178344add5d1-pr-283	✅