Don't run kanban assignment on forks

Without this condition, the kanban assignment would be run on every
CONSUL fork and that would result in unpredictable results since they
would try to write on our kanban, and they don't have permission to do
so.

Note that, much to my dismay :D, the code only works if we use single
quotes in the name of the repository owner; it doesn't work if we use
double quotes.

.github/workflows/kanban.yml

@@ -7,6 +7,7 @@ env:
 
 jobs:
   assign_one_project:
+    if: github.repository_owner == 'consul'
     runs-on: ubuntu-latest
     name: Assign new pull requests
     steps: