forked origin requires a forked destination

Fixes: https://code.forgejo.org/actions/cascading-pr/issues/10

action.yml

@@ -21,7 +21,7 @@ description: |
   The `update` script is expected to be found in the origin repository
   running the PR. It is given four arguments:
 
-    * A directory in which the destination repository is checked-out
+    * A directory in which the destination repository (or a fork) is checked-out
       on the base branch
     * A file with the JSON describing the pull request in the
       destination repository
@@ -36,6 +36,9 @@ description: |
   When the PR is from a forked repository, the `update` script is checked out from
   the default branch instead of the head branch of the fork.
 
+  If the fork of the destination repository is specified and it does
+  not exist, it is created.
+
 inputs:
   origin-url:
     description: 'URL of the Forgejo instance where the PR that triggers the action is located (e.g. https://code.forgejo.org)'
@@ -55,6 +58,8 @@ inputs:
   destination-repo:
     description: 'the repository in which the cascading PR is created or updated'
     required: true
+  destination-fork-repo:
+    description: 'the fork of {desitnation-repo} in which the {destination-branch} will be created or updated'
   destination-branch:
     description: 'the base branch of the destination repository for the cascading PR'
     required: true
@@ -102,6 +107,7 @@ runs:
                         --origin-pr "${{ inputs.origin-pr }}" \
                         --destination-url "${{ inputs.destination-url }}" \
                         --destination-repo "${{ inputs.destination-repo }}" \
+                        --destination-fork-repo "${{ inputs.destination-fork-repo }}" \
                         --destination-token "@$destination_token" \
                         --destination-branch "${{ inputs.destination-branch }}" \
                         --update "${{ inputs.update }}" \