In order to specify which terraform version to run you can do so in the digger_workflow.yml file. This example shows how you can do it for terraform: 
jobs:
  destroy:
    runs-on: ubuntu-latest
    steps:
    - name: digger destroy
      uses: diggerhq/digger@vLatest
      with:
        setup-terraform: true
        terraform-version: v1.5.5
      env:
        GITHUB_CONTEXT: ${{ toJson(github) }}
        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
And similarly for OpenTofu: 
jobs:
  destroy:
    runs-on: ubuntu-latest
    steps:
    - name: digger destroy
      uses: diggerhq/digger@vLatest
      with:
        setup-opentofu: true
        opentofu-version: v1.6.1
      env:
        GITHUB_CONTEXT: ${{ toJson(github) }}
        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
When you use OpenTofu, you also need to enable opentofu: true in the project settings in digger.yml.