How do I pass variable as argument to CLI parameter in YAML file?

Question

I am trying to run detekt only on changed files in a pull request. The following doesn't work when I try to pass CHANGED_FILES as an argument to the input parameter for detekt.

# This workflow performs a static analysis of your Kotlin source code using
# Detekt.
#
# Scans are triggered:
# 1. On every push to the main branch
# 2. On every Pull Request targeting the main branch
name: detekt
## 2
on:
  # Allows you to run this workflow manually from the Actions tab
  workflow_dispatch:

  # Triggers the workflow on push or pull request events but only for default and protected branches
  push:
    branches:
      - main
  pull_request:
    branches:
      - main

# A workflow run is made up of one or more jobs that can run sequentially or in parallel
jobs:
  # This workflow contains a single job called "detekt"
  detekt:
    # The type of runner that the job will run on
    runs-on: ubuntu-latest
    steps:
      # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
      - name: "checkout"
        uses: actions/checkout@v3
      # Determines changed files
      - name: "changed_files"
        run: |
          git fetch origin main:refs/remotes/origin/main
          export CHANGED_FILES=$(git --no-pager diff --name-only origin/main HEAD | tr '\n' ',' | sed 's/\(.*\),/\1 /')
          echo "Changed files: $CHANGED_FILES"
      - name: "detekt-all"
        uses: natiginfo/[email protected]
        env:
          CHANGED_FILES: ${{ CHANGED_FILES }}
        with:
          args: --fail-fast --config detekt-config.yml --plugins detekt/detekt-formatting-1.21.0.jar --input $CHANGED_FILES

I get this error:

Exception in thread "main" com.beust.jcommander.ParameterException: Provided path '$CHANGED_FILES' does not exist!

I tried multiple other ways, but to no avail. I'm not familiar with YAML and I'm trying to figure out the correct syntax to do this, but I could not find any guidance about this specifically. How do we correctly pass it as an argument to a parameter instead of it being taken as the path itself? If I'm going about this the wrong way, then please let me know how to achieve this differently.