fix: bitbucket service pull request build status from different source

The commit 7ac886d switched to the newer repository commit build
status api. This introduced a regression in the case the PR source
repository is not the same as the target, for example a person fork.
In this case the bitbucket api created from the PR head will point to
the fork repo and may result in a 401 error if the credentials don't
have the necessary permissions.

```
ERROR: Could not send notifications
com.cloudbees.jenkins.plugins.bitbucket.api.BitbucketRequestException: HTTP request error.
Status: HTTP/1.1 401
Response: {"errors":[{"context":null,"message":"You are not permitted to access this resource","exceptionName":"com.atlassian.bitbucket.AuthorisationException"}]}
```

This change will force the bitbucket api instance to be the target repo
which is the appropriate target for the notification. The old api was
commit hash centric, not project centric, so the api client owner/repo
didn't matter.

This change also reverts changes introduced by jenkinsci#954 to the refName
parameter. Since notification is on the targget repo, the head ref
branch may not exist on the target and result in no status update.

No changes were made to the Bitbucket Cloud code path.

Author: cronik

src/main/java/com/cloudbees/jenkins/plugins/bitbucket/impl/notifier/BitbucketBuildStatusNotifications.java

@@ -230,18 +230,18 @@ private static void sendNotifications(BitbucketSCMSource source, Run<?, ?> build
              * https://community.atlassian.com/t5/Bitbucket-questions/Re-Builds-not-appearing-in-pull-requests/qaq-p/1805991/comment-id/65864#M65864
              * that means refName null or valued with only head.getBranchName()
              *
-             * For Bitbucket Server, refName should be "refs/heads/" + the name
-             * of the source branch of the pull request, and the build status
-             * should be posted to the repository that contains that branch.
-             * If refName is null, then Bitbucket Server does not show the
-             * build status in the list of pull requests, but still shows it
-             * on the web page of the individual pull request.
+             * For Bitbucket Server, refName is optional.
+             * https://developer.atlassian.com/server/bitbucket/rest/v905/api-group-builds-and-deployments/#api-api-latest-projects-projectkey-repos-repositoryslug-commits-commitid-builds-post
+             * The PR head revision may point to a different source repository, for example a personal fork. We will
+             * be notifying the PR target repo status where the head branch may not exist, so a null refName will
+             * suffice.
              */
+            refName = null;
             bitbucket = source.buildBitbucketClient(head);
-            if (BitbucketApiUtils.isCloud(bitbucket)) {
-                refName = null;
-            } else {
-                refName = "refs/heads/" + head.getBranchName();
+            if (!BitbucketApiUtils.isCloud(bitbucket)) {
+                // For  Bitbucket Server, notify the target of the PR. Head may point to a source repo that the
+                // credentials don't have access to resulting in a 401 error.
+                bitbucket = source.buildBitbucketClient();
             }
         } else {
             listener.getLogger().println("[Bitbucket] Notifying commit build result");