append to gh output

.github/workflows/sphinx.yml

@@ -30,7 +30,7 @@ jobs:
         run: |
           import os
           lib_name = "${{ github.repository }}".split("/")[1]
-          with open(os.environ.get("GITHUB_OUTPUT")) as gh_out:
+          with open(os.environ.get("GITHUB_OUTPUT"), "a") as gh_out:
               gh_out.write(f"lib-name={lib_name}\n")
 
       - name: Upload docs build as artifact