Update release scripts for new hackage-doc-tarball job

Also remove the GHC_TREE variable from the upload script as it violates the principle that we need to be able to trace back all artifacts to a release job. GHC_TREE could be pointed at any directory which would lead to docs not being created via the CI pipeline.

rel-eng/fetch-gitlab-artifacts/fetch_gitlab.py

@@ -33,6 +33,7 @@ def job_triple(job):
 
         'source-tarball': 'src',
         'doc-tarball': 'docs',
+        'hackage-doc-tarball': 'hackage_docs',
     }
     if job.name in bindists:
         return bindists[job.name]
@@ -94,6 +95,10 @@ def fetch_artifacts(release: str, pipeline_id: int,
                     logging.info(f'extracted docs {f} to {dest}')
                 index_path = destdir / 'index.html'
                 index_path.replace(dest / 'index.html')
+            elif job.name == 'hackage-doc-tarball':
+                dest = dest_dir / 'hackage_docs'
+                logging.info(f'moved hackage_docs to {dest}')
+                (destdir / 'hackage_docs').replace(dest)
             else:
                 dest = dest_dir / f'ghc-{release}-{triple}.tar.xz'
                 if dest.exists():