When and Unless Step on GitHub Fork Pull Request (PR) but not on Primary Repo?

Build Environment
1

Struggling to skip a single step when a GitHub Fork opens a Pull Request. The gist. I do not want to expose secrets to PR builds, but I want to do everything else. When I accept the PR from the Fork, I want to run the entire build (which includes using secrets for security scanning).

What am I doing wrong?

I’ve used {<<<—} to indicate what I “think” I should be doing but probably doing wrong. More attempts on conditions at the end.

sample config.yml
jobs:
main:
parameters:
image:
type: string
circle_pr_number:
type: string
default: $CIRCLE_PR_NUMBER <<<— I couldn’t figure out how to use the Built-in environment variables in a conditional statement; my hack, Appears that $CIRCLE_PR_NUMBER is a string and not a number.

docker: 
  - image: <<parameters.image>>

steps: 
  - checkout
  - run: echo <<parameters.circle_pr_number>>  <<<--- Outputs the correct value of the PR (e.g., 302)
  - unless:
      condition:  
        matches: <<<--- latest attempt was to use a regex
          pattern: \d
          value: <<parameters.circle_pr_number>>
      steps:
        - run: echo "condition met"

OTHER ATTEMPTS

      condition:  <<parameters.circle_pr_number>>

      condition:  
                 equals: [ "", <<parameters.circle_pr_number>> ]

      condition:  
                 equals: [ 301 , <<parameters.circle_pr_number>> ]

Stumped.