Github Action doesn't checkout submodule of action repository

[JohannesK71083]

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Stackoverflow question

On Github actions, submodules of the action repository aren't checkout out, instead only an empty folder remains and it's impossible to load the submodules manually, because github action deletes/doesn't create the .git folder (so it only checks out the files and not the repo). I also tried publishing it to Github Marketplace to see if that would fit it, but it didn't. (After that I deleted it again from the marketplace.)
How can I fix that?:

File Setup:

main repo:
|   action.yml
\--- submodule

submodule repo:
|   test.txt

main_repo/action.yml:

name: "Submodule Test"
description: ""

runs:
  using: "composite"
  steps:
    - name: execute test
      shell: cmd
      run: |
        tree /F /A ${{ github.action_path }}

.../.github/workflows/test.yml

name: Submodule Test
on:
  workflow_dispatch

jobs:
  Pyinstaller:
    runs-on: windows-latest  # same problem on ubuntu-latest
    steps:
      - name: execute pyinstaller_action
        uses: "JohannesK71083/actions_submodule_main@main"

Github actions output:

D:\A\_ACTIONS\JOHANNESK71083\ACTIONS_SUBMODULE_MAIN\MAIN
|   .gitmodules
|   action.yml
|   
\---submodule

Links to the repos:
main repo
submodule repo
action run logs

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