move checkout action to separate path

action.yml

@@ -1,22 +0,0 @@
-# SPDX-License-Identifier: GPL-3.0-or-later
-name: 'Checkout git repository'
-author: 'Manuel Friedli'
-description: |
-  Checkout a git repository.
-
-  It performs a shallow checkout and does not require NodeJS to run.
-
-inputs:
-  hostname:
-    description: The hostname of the forge we're checking out from.
-    default: "gittr.ch"
-runs:
-  using: "composite"
-  steps:
-    - run: |
-        set -eu
-        git init --quiet --initial-branch main
-        git remote add origin "https://${{ forge.token }}@${{ inputs.hostname }}/${{ forge.repository }}.git"
-        git fetch --quiet --no-tags --depth=1 origin "${{ forge.sha }}"
-        git checkout --quiet "${{ forge.sha }}"
-      shell: bash