Configure the matrix job directory settings using GitHub labels.

Create a json file in the following format. Specify the label name as key and target directory as value.
The config file name or directory can be specified, but .deploy_target.json is used by default.

{
    "target:develop": [
        "envs/development"
    ],
    "target:staging": [
        "envs/staging"
    ],
    "target:production": [
        "envs/production"
    ],
    "target:all" : [
        "envs/development",
        "envs/staging",
        "envs/production"
    ]
}

If the jq and gh commands are installed, the following commands are useful.

export REPO=<OWNER>/<REPO_NAME>
export COLOR=5319E7
cat .deploy_target.json | jq -r 'keys | .[]' | xargs -I @ gh label create @ --color $COLOR --repo $REPO

ref. https://cli.github.com/manual/gh_label_create

Example

$ cat .deploy_target.json | jq -r 'keys | .[]' | xargs -I @ gh label create @ --color $COLOR --repo $REPO
✓ Label "target:all" created in ponkio-o/select-target-action
✓ Label "target:develop" created in ponkio-o/select-target-action
✓ Label "target:production" created in ponkio-o/select-target-action
✓ Label "target:staging" created in ponkio-o/select-target-action

Please refer to following doc.
https://docs.github.com/en/issues/using-labels-and-milestones-to-track-work/managing-labels#creating-a-label

GitHub Actions are configured as follows:

name: Terraform PR check

on:
  pull_request:
    types: [opened]
    branches:
      - main

jobs:
  select_target:
    name: Select target
    runs-on: ubuntu-latest

    outputs:
      targets: ${{ steps.sta.outputs.targets }}

    steps:
      - name: Checkout
        uses: actions/checkout@v3

      - name: Select target
        id: sta
        uses: ponkio-o/select-target-action@main

  plan:
    needs: [select_target]
    name: Plan
    runs-on: ubuntu-latest

    strategy:
      matrix:
        target: ${{fromJson(needs.select_target.outputs.targets)}}

    steps:
      - name: Checkout
        uses: actions/checkout@v3

      - name: Setup terraform
        uses: hashicorp/setup-terraform@v1

      - name: Terraform plan
        working-directory: ${{ matrix.target }}
        run: terraform plan -input=false -no-color
...

The directories set in the key of the given label are merged and returned as an array.

Assign labels to the Pull Request. You can also select multiple labels.

If you have directories that you want to run by default, you can set them to the "default" key. It can be used in conjunction with other labels, and if valid labels are selected, it will take precedence.

{
    "default" : [
        "envs/development",
        "envs/staging",
        "envs/production"
    ]
}

All inputs are optional.

The working directory outputs as an array.

Example:

["envs/development","envs/staging","envs/production"]