Fix JENKINS-69719: Fetch PR data from target repo (#67)

Jenkins token permissions potentially invalid to access source repo.
This is the cause of issue [JENKINS-69719](https://issues.jenkins.io/browse/JENKINS-69719).

All required Gitea Pull Request data can be accessed via target repo.
Replace access to potentially inaccessible source repository by references in target repository Pull Request head.

### Testing done

The change affects token/credential permission scope on the Gitea server.
With the changed source for data access, the `MultiBranchPipeline` scan completes.

Jenkins can pull/checkout (moving) head of pull request.
Resolution to commit hash is done independently in body of `GiteaSCMFileSystem` constructor.

Author: astos-marcb

src/main/java/org/jenkinsci/plugin/gitea/GiteaSCMFileSystem.java

@@ -131,9 +131,9 @@ public SCMFileSystem build(@NonNull SCMSource source, @NonNull SCMHead head, @Ch
             String repository;
             String ref;
             if (head instanceof PullRequestSCMHead) {
-                repoOwner = ((PullRequestSCMHead) head).getOriginOwner();
-                repository = ((PullRequestSCMHead) head).getOriginRepository();
-                ref = ((PullRequestSCMHead) head).getOriginName();
+                repoOwner = src.getRepoOwner();
+                repository = src.getRepository();
+                ref = head.getName();
             } else if (head instanceof BranchSCMHead) {
                 repoOwner = src.getRepoOwner();
                 repository = src.getRepository();